Web Parts, Why-eeee

Web Parts are things people have written that plug into the more front-end portions of the Sharepoint infrastructure. They are used to control and display your back-end information. Although there are a plethora of them, in practice you will be using only one or two. These are the Content Query Webpart, in Sharepoint 2007, and Are All Of These Actually The Same Thing? in Sharepoint 2010.

Sharepoint 2010: Drop A Block In the Page and Call It A Day
All lists (everything is a list in sharepoint) can be displayed as webparts, which are functionally tiny windows within windows with their own display settings. Each list you have on a subsite automatically generates its own page, with a webpart in it, which displays the content of the list according to settings you choose in the toolbar, hopefully while sober and lacking the flu.

Document libraries, calendars, everything - you set what to display on their page, and then all future views of the list will just load those settings, correct?

False.

Those settings are not centralized, they are page-specific. That means that every time you need to load a given view of a list into a page, you're gonna be setting how you see that list. Even if it is an extremely specific set of preferences about, say, group selection and display.
JOB SECURITY.

There are ways around this - for example, setting everything on the one calendar page and then exporting the webpart with all its settings, and loading it to whatever page you end up with - but in practice, this will lead to you storing hundreds of XML files with various almost indistinguishable webpart settings, which you use in two places on your site; the first where you made it, the second on the landing page for the subsite. Not particularly efficient in real terms.

So in practice,  you get to hand-edit every homepage, and prepare to answer a lot of questions about where the documents are actually living. This is marginally better than managing check-ins, and slightly worse than sorting out version control.

Performance Management Easy Achievement High Score
Do you need to game a performance management system in advance of Spring Review? Oh shit you might. Great. Customize twenty of the damn things and stuff 'em in a folder and then you suddenly have quantifiable job metrics for data-driven review. Well done you. Your horrific cynicism will carry the day yet.