A tracking tag lets you add information to a hit about where the visitor came from. For example, the "Take The Tour" button on the home page has a source=front-page tag on it. Here are the hits on the tour link, shown by source tags:
Period
front-page
tour-top-tab
business-card
N/A
front-page"
tour-top-tab"
booktrakr
tour-top-tab2121121121212.1
tour-top-tab999999.1 union select unhex(hex(version())) -- and 1=1
tour-top-tab99999' union select unhex(hex(version())) -- 'x'='x
tour-top-tab99999" union select unhex(hex(version())) -- "x"="x
tour-top-tab and 1=1
tour-top-tab and 1>1
tour-top-tab' aND BeNChMaRK(2999999,Md5(NoW())) AnD '1
tour-top-tab' AnD sLeep(3) ANd '1
tour-top-tab' and 'x'='x
tour-top-tab" and "x"="x
tour-top-tab' and 'x'='y
tour-top-tab" and "x"="y
tour-top-tab'&&BeNChMaRK(2999999,mD5(NOW()))&&'1
tour-top-tab or (1,2)=(select*from(select name_const(CHAR(107,107,122,99,72,108,72,80,111,97),1),name_const(CHAR(107,107,122,99,72,108,72,80,111,97),1))a) -- and 1=1
tour-top-tab' or (1,2)=(select*from(select name_const(CHAR(107,107,122,99,72,108,72,80,111,97),1),name_const(CHAR(107,107,122,99,72,108,72,80,111,97),1))a) -- 'x'='x
tour-top-tab" or (1,2)=(select*from(select name_const(CHAR(107,107,122,99,72,108,72,80,111,97),1),name_const(CHAR(107,107,122,99,72,108,72,80,111,97),1))a) -- "x"="x
tour-top-tab'&&sLEEp(3)&&'1
businesscard
Total
Total
3209
1490
935
153
28
20
4
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
5859
Tags help you figure out where to focus your efforts. Do you get more traffic through Twitter or Facebook? From the front of an e-book or the back? If your links are properly tagged, you can tell.