What's the difference between a leecher and a seeder?

A leecher has either some or none of the files in a torrent. They work the same except a seeder isn't downloading anything from others. A seeder has the whole file, and leaves the torrent client running to help others filling in the blanks that other leechers might not have. Leechers only have bits and pieces of the whole file.

Can a seeder upload a file faster than a leecher?

In reality, its possible that one seeder might be able to upload the file at a much faster rate than another. Or, that one leecher is able to download a file at a much faster rate than another. It just depends on the internet connection.

How does a leecher work?

Leechers only have bits and pieces of the whole file. Once your download is complete you go from a leecher to a seeder automatically. Most people close the torrent when it's done because uploading is the riskiest part of torrenting, hence why people are always encouraged to seed as a way to "pay" for the free content.