I'll give you a basic intro to worksets and provide a few tips we've learned.
When you have a large project and you want multiple users editing a project at the same time you have to implement Worksets because Revit typically keeps all project data in a single project file unlike Autocad where we typically separate the project into multiple files.
Go to File... Worksets... to activate the Worksets feature in your project file. Revit will by default divide the project into 2 worksets "Shared Levels and Grids" and "Workset 1." All of your model objects will initially reside in Workset 1. You can further divide Workset 1 into other user-created worksets at some future time.
Save your new project file to a central server where everyone can access it. The file you're working in is now the Central File. Before making any more changes to the project file go to File... Save As... and save a local copy of this central file to your desktop (each user should save a local copy onto their desktop before they start working on a project file). You now have a Central File on the server and a duplicate Local file that you will be working in. If you ever wanted to make your Local copy a Central File goto Options when you Save As and check the box to "Make this the Central location after save."
Now from your local copy go to File... Worksets (there is also a worksets toolbar available). When a workset is opened it is loaded into memory and visible in all views. If you find the project is too slow with all the worksets opened then just close the worksets you won't be working on, but remember to open all the worksets again before plotting.
When you make a workset Editable you are the sole owner of a workset. You are the only person who can edit the workset.
When you select an object that's in a Non Editable workset you'll see a blue puzzle icon. Click on the icon and Revit will let you Borrow the element provided there are no users who Own the workset you're borrowing from.
You can rename and edit worksets that are User-Created. The three other workset types View Worksets, Family Worksets, and Project Standards are automated worksets that you can only turn on/off.
Here are a few tips we've learned in implimenting Worksets:
1. Enable worksets when you expect to have multiple users accessing the project file at the same time or when you expect the project is going to get so big that keeping the whole project open is going to be a strain on your workstations resources.
2. If you're going to open and close worksets for increased speed it is best to initiate the worksets feature as soon as possible. If you don’t initiate worksets early on you’re going to have to assign every model object to a workset after they’ve already been created and that can be a lot of unnecessary work.
3. If you are just going to use worksets so multiple users can access one file then the worksets feature can be enabled at any time. I would advise against adding more than the (2) default worksets unless the project file is just too slow and you're willing to spend half your time managing objects and the worksets they reside on.
4. When ever possible BORROW elements instead of OWNING worksets. We find that no matter how you divide your worksets users inevitably need to work on different parts of the same workset.
5. If you can help it do not use worksets to control visibility of objects in a view. Worksets are for closing large parts of the model when loading them is a strain on the resources of your workstation and for making project files accessible to multiple users.
There is a lot more to know about worksets but hopefully this gives you a place to start.
EDIT (4/18/2014): This post was, and continues to be, the most popular post on my blog so I thought I'd add a few additional tips.
6. If you have a site model with 10 buildings linked into it, consider putting each linked file on its own workset. When I'm working on a site model I always open "by selective worksets" to open just Workset 1, Shared Levels and Grids, and perhaps one of the buildings I'm modeling sitework around.
7. Keep in mind that when putting a linked model on a workset it has a setting for the workset in both instance and type parameters. Might be a good thing to know before you start.
8. There was a question about enabling worksets in a project template. Revit does not support worksets in template files BUT you can do all of your template work in a workshared project file (say if you've assigned multiple authors to one template), then make that project file a template file (detach and discard worksets) when you are ready to publish the template. More on AUGI
9. Never delete all the worksets (yes it has happened). You have to detach without preserving worksets and recreate a central with the "restored and default" worksets.
10. You can Own a workset with a special character username to lock everyone out of it (to prevent tip #9 for example).
11. Hint: If you are having performance issues consider that Local and central files are usually not the same size. Compacting one or the other or both are all options.