SAP is an Enterprise Resource Planning software application. Even more simply, it is a software application. Just like any other software application, it must reside on an operating system which provides services to the application for it to run. SAP can function on top of Windows and many versions of UNIX (Linux, AIX, Solaris, HPUX).
"What is SAP Basis?"
In the early days, SAP had just one component, SAP R/3 (today called SAP Enterprise Core Component or Enterprise Central Component or ECC). SAP R/3 sat layered on top of a more basic technical layer, often refered to as "SAP Basis". SAP Basis acts as a filter (technically called an abstraction layer) between the actual business logic in SAP R/3 and the specifics of the operating system and database underneath. In this way SAP business programmers could focus on writing business logic (in SAP's proprietary ABAP language) and not have to worry whether or not it would work on the various permutations of operating system and database. Basis handled all that. Long before JAVA was even conceived, SAP mastered the concept of "write once, run many" that JAVA promised but has yet to fully deliver. You can think of an SAP application as a layer cake: hardware -> operating system -> database -> SAP basis -> SAP application. The Basis layer and the SAP application layer are bundled up and can be placed on top of many different permutations of HW/OS/DB and still function as intended. No need to recompile or rewrite code to change HW/OS/DB.
"What is an SAP Module?"
SAP R/3 (now called SAP ECC, see above) contains a staggering amount of business logic. The whole point of a enterprise resource planning system is that it runs an entire business. That means that *all* common business functions: Sales and Distribution (SD), Financials and Controlling (FICO or just FI), Human Resources (HR), Materials Management(MM), and Production Planning (PP) to name some of the most popular functions. Each of theses business functions can be turned on and off individually and is referred to as a module. Even within a module, you can configure just a piece of the module to work and ignore the rest of the functionality if you like. So, basically, a Module is a convenient term used to refer to a subsection of the functionality of a component.
"What is an SAP Component?"
At some point, SAP started to branch out from the business processes that almost every company needs (functionality within ECC) and create separate, stand alone, components that only specialized companies need. You can think of these components as extreme versions of the modules. For example, extreme Sales and Distribution is Customer Relationship Management. Extreme Materials Management is Supplier Relationship Management. Extreme Production Planning is Advanced Planning and Optimization (APO). These components were built so they could function standalone or in conjunction with SAP ECC. The idea is that every company needs SAP ECC to cover core processes but if your company needs more, then you can augment with the more specialized components.
"What is SAP NetWeaver?"
Originally all of the components were built on top of the same ABAP Web Application Server (Basis), but as SAP's offerings became more sophisticated, other basic infrastructure came into play. SAP purchased an JAVA company and a Portal company. SAP also developed it's own Data Warehouse. Since all of these very basic tools were pretty much needed at all customers, they were bundled together under the SAP NetWeaver umbrella (along with a few more items, like eXchange Infrastructure, XI -- later renamed Process Infrastructure, PI). The NetWeaver tools are basically a toolkit that you can use to augment or enhance the business functionality delivered by a component. Since NetWeaver provides the plumbing underneath all of the components, it is often interchangeably referred to as SAP Basis in reference to the original ABAP stack that was the foundation of SAP R/3.
"What does an SAP NetWeaver System Administrator do?" or "What does an SAP Basis person do?"
As discussed earlier, SAP NetWeaver is a collection of tools that lies underneath all SAP Components. You can think of that toolset (more specifically the ABAP and JAVA stacks) as an operating system that sits on top of whatever operating system lies underneath any given SAP component. Many of the activities that an SAP NetWeaver System Administrator does day-to-day are similar to the activities of an OS system admin, a database admin, and/or network admin. The SAP Basis person performs his/her activities from within the SAP software as well as at the OS and DB layers/interfaces.
In a small shop, a typical SAP basis person might do the following tasks: create users/assign roles (within SAP), run backup, check db/os space utilization, add space if necessary, install SAP software, configure SAP parameters, monitor CPU/Memory/disk space/performance, configure connectivity between SAP components or SAP/nonSAP components, configure printing queues/printers, and/or sap software change management (aka Transports or Transport Management). Not an exhaustive list, but you get the idea. For the most part these are all activities that would be familiar to a UNIX/Windows OS admin, a DBA, or a network admin. Most SAP Basis folks start their careers as OS sysadmins and/or DBAs. A smaller number start life as network administrators.
In very small shops, the SAP Basis Admin often wears all four hats (os admin, dba, network admin, and SAP admin). As you go up in company size, you see more specialization. Many companies have a separate dedicated admin for each function: OS admin, DBA, network admin, and SAP Basis admin. As you go further up the scale in size, you'll find those responsibilities spread even thinner: one or more OS admins, SAN admin, one or more DBAs, one or more SAP Basis admins, one or more dedicated SAP Security personelle, etc. I'll cover Basis specializations in a separate blog post.
"You can't get hired as an SAP Basis Admin without experience, so how do you get experience?"
There are several paths, but these two are the most common: 1) Company already runs SAP and has experienced staff. Hard workers from desktop support/os admin/dba pool get promoted to SAP Basis group. 2) Company does NOT have SAP, but buys it. Existing OS/DBA/network admins get tasked with helping implement SAP with help of consultants. You'll not that there is NOT a group three in which a recent college graduate pays a chunk of money to get SAP certification and then gets hired to do SAP Basis work. It happens, but VERY infrequently. Mostly companies want to know that you can function as a normal admin before training you to be an SAP Basis admin. Thousands of people get their start as SAP Basis Admins every year through the first two routes. This means that there are PLENTY of experienced people available for every job that is posted for SAP Basis work. I can't stress this enough: if you have no work experience as an SAP Basis admin, you are wasting your time applying for SAP Basis admin openings. The folks you are competing against for those jobs typically have 8 or more years as an admin with at least 2 or 3 of those years doing SAP Basis work specifically. If you have no experience and would like to have a career as an SAP Basis person, go get hired as an OS admin, a DBA, or a network admin at a company that already runs SAP. That is, by far, your best chance to eventually get into SAP.
"What role does SAP Certification play in this process?"
If you read up on SAP Certification, you'll see that SAP recommends certification for experienced folks only. That means, get a job as an admin, then get a job as an SAP admin and get trained. Work as an SAP admin for a a while... THEN get certified. Certification is not a golden pass to your dream job if you have no existing work experience (see above), but it CAN provide a key differentiator between two experienced candidates applying for the same junior to mid level Basis job. Some consulting companies require all of their consultants to be certified to give their customers a better feeling about hiring the consulting company. Once you are very experienced, the certification loses some of it's power again as you can generally get hired by referencing work you've done in the past.
I hope this gives you a good idea of what SAP Netweaver (Basis) is all about and how to start off your career if you decide SAP Basis is your path.