Armor set - WoWWiki - Your guide to the World of Warcraft
1. You get those tier tokens by killing some bosses in raids, for example to get the chestpiece you are talking about you must kill The 4 horsemen in Naxxramas, you must check if it dropped the token needed for your class.
2. They are kinda emblems, is just a currency, you just use it to buy your armor set, if you have a chest token you can buy a chest tier piece, if you have a helm token you buy a helm piece, can be dps or prot pieces.
Not all tier tokens are the same, they change with each new raid instance and the version of the raid (10m or 25m)
Tier7 came with Naxxramas 10m, and its ''nickname'' to recognize this tier is ''Heroes''
Tier7.5 is from Naxxramas too but in its 25m version, this one is called ''Valorous''
Tier8 is from Ulduar 10m, this is called ''Valorous''
Tier8.5, Ulduar 25m, ''Conquerors''
Tier9, Trial of the Crusader, ''Conquerors'', it changed a little bit since you get this set with Emblems of Triumph
Tier9.25???, Trial of the Crusader, ''of Triumph'', you can buy this set with Emblems of Triumph and a Trophy of the Crusade that drops from each boss in ToC25m and by doing ToC10m/25m in its Heroic version.
Tier9.5???, Trial of the Crusade 25m Heroic Mode, you get Tokens for this set by defeating this instance in its Heroic Version
also i found this thread that can help you a bit too.
World of Warcraft - English (NA) Forums -> What does tier mean?