But very, very simply:
Seeding means either you have uploaded your own torrent and are sharing it with other people or:
You have completed a download of someone else's torrent and you have left your torrent client open to help others by sharing.
Leeching means that you are downloading someone else's torrent and haven't completed it yet. Once you have got the complete file, you become a seeder if you let other people have a share of your completed download.