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
tour-top-tab"
front-page"
tour-top-tab'123
front-page'123
' OR 1=1
' AND EXTRACTVALUE(1,CONCAT(0x5c,USER()))--
' AND GTID_SUBSET(CONCAT(0x7e,USER(),0x7e),1)--
' AND (SELECT 1 FROM(SELECT COUNT(*),CONCAT(USER(),FLOOR(RAND(0)*2))x FROM information_schema.tables GROUP BY x)a)--
' AND (SELECT 1 FROM(SELECT NAME_CONST(USER(),1),NAME_CONST(USER(),1))a)--
'
' OR 'x'='x
' UNION SELECT 1,2,3--
' UNION SELECT null--
' UNION SELECT null,concat(login,':',password) FROM users--
' UNION SELECT table_name,column_name FROM information_schema.columns--
' UNION SELECT @@version,user(),database()--
' UNION ALL SELECT null,schema_name FROM information_schema.schemata--
'nvOpzp; AND 1=1 OR (<'">iKO)),
"><h1>BlackHatCracker PRO hacked by hacklaplanete</h1>tour-top-tab
tour-top-tab'_
"><h1>BlackHatCracker PRO hacked by hacklaplanete</h1>front-page
front-page'_
Total
Total
3226
2130
912
30
30
27
9
9
6
6
6
6
6
6
6
6
6
6
6
6
6
1
1
1
1
1
6456
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.