How to understand user agents?

Can somebody please help me understand user agents? for example, what browser & browser version are these guys using?

[TABLE="width: 517"]
[TR]
[TD]Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727)
[/TD]
[/TR]
[/TABLE]
[TABLE="width: 589"]
[TR]
[TD]Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/536.11 (KHTML, like Gecko) Chrome/20.0.1132.57 Safari/536.11

[TABLE="width: 589"]
[TR]
[TD]Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.28) Gecko/20120306 Firefox/3.6.28

[/TD]
[/TR]
[/TABLE]
[/TD]
[/TR]
[/TABLE]

I'll take a guess...?

1. Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727)
2. Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/536.11 (KHTML, like Gecko) Chrome/20.0.1132.57 Safari/536.11
3. Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.28) Gecko/20120306 Firefox/3.6.28

Click to expand...

1. no idea
2. Guy is using Chrome version 20.0? but dunno why safari is showing
3. Guy is using firefox version 3.6.28

Please help!

http://lmgtfy.com/?q=understand+user+agent+string

The first link linking to microsoft site explains it very well.