For the meaning of each icon type in the Peers tab, check this topic (the Connection Status paragraph): Detailed Info Pane - Peers
As for your last question, to put it the simple way, know this: peer-to-peer transfers are very different from client-server transfers.
In client-server transfers (e.g. you download some file from a FTP or HTTP server such as Rapid Share) what happens is that you contact the server, request the file and once the download started, it all depends on mostly 3 factors:
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your max download speed
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the server’s degree of congestion and also its upload speed (but usually that exceeds by far your line capacity unless it’s a personal web server, so it’s usually not a real issue)
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the degree of congestion in the network path between you and the server
In peer-to-peer transfers you don’t have servers. You download only from peer computers of other people such as yourself.
Especially in BitTorrent, there’s some quite complex algorithms which govern the criteria based on which peers establish and tear down connections between themselves. If you want to know more about it, I suggest you to go and read the BitTorrent protocol specifications, to get a feel of it.
But as a thumb rule know that you’ll get peers more or less alike you (i.e. proportional to your upload capability). Meaning that while downloading, peers with high upload bandwidth will end up teaming with other high upload peers (occasionally you will get some too, if they don’t find better partners).
But the reverse can also be true, meaning that you have a high upload line and on some torrents you end up with lousy speeds, simply because there aren’t any fast uploading peers in the swarm of that particular torrent.
So, if you really want to understand what’s going on behind the scene you need to go and read quite a bit.
If you’re just worried if that’s normal behavior, well, the short answer is: Yes. Sure, from case to case you’ll have sometimes stable peers with steady upload and other times very dynamic peers with varying speeds, depending on the swarm composition but that doesn’t necessarily indicate a problem on your end.
If you have a green status light and have your client settings at their best values, then that’s all you need to and CAN do.