In some ways it's like a Facebook for Professionals (if you use Facebook). I'm a member in it as well as a lot of my coworkers.
It allows you to connect with coworkers past & present as well as friends and family and business associates. From it people carry on discussions in somewhat of an instant messenger type of a format. You can also set it up to get notifications when new messages are added to groups you might join or when your "connected people" might update their profile.
You can also list education, past work experience, etc. and build it into an online resume for yourself that can be reviewed by everyone you've connected to. You can also request recommendations from others. Through all this it can be a good source of reference if your job hunting. Better for those that are looking to "work for the man" than it is for self employed folks, as it gives you a reference you can share with HR departments.