In simple words, Seeders are people who are giving that file to you and Leechers are taking that file (downloading) from them. So, You are probably a Leecher.
There is lot of confusion over seeders. Though they are offering the file to you but they will be called true seeders if they have 100% of the file to be hosted. These seeders have complete achieve and now only uploading to the other leechers.
On the other hand leechers are those who has not completed the torrent file and are downloading from the other leechers and seeders.
leech = downloading from others.
seed = uploading for others to download.
Common sense says that if there are more seeders then the downloading speed is optimum but if there are more leechers then it might result in slow downloads. So always go for files with more seeders or more seed/leech ratio.
The idea behind seeders/leechers is that you aren't relying on just one person or place to download your file. The more leechers that are on a file , the more strain / bandwidth is being used from the seeders. Try to find an equally balanced file i.e equal number of Seeders and leechers. Best situation is more seeds than leechs.