You can think of LinkedIn as the high-tech equivalent of going to a traditional networking event where you go and meet other professionals in person, talk a little bit about what you do, and exchange business cards. It's like one big virtual networking event.
On LinkedIn, you network with people by adding them as 'connections,' similar to how you'd make a friend request on Facebook. You converse via private message (or available contact information), and you have all of your professional experience and achievements laid out in a neatly organized profile to show off to other users.
LinkedIn is similar to Facebook in terms of its layout and broad feature offering. These features are more specialized because they cater to professionals, but in general, if you know how to use Facebook or any other similar social network, LinkedIn is somewhat comparable.