Rushing: rushing is a tactic used in online FPS games (like crossfire) where you run or "rush" at the opponents straight from the start getting as many kills and causing as much damage as possible.
The best gun for rushing is probably the Mp5. It has a high fire rate and an uber fast reload. Not to mention you run as fast with an Mp5 as you do with a knife...
Rushing is more useful in maps that are smaller with many twists and corners. Good rushing relies on using corners to your advantage and your speed to cause LOTS of damage.
Tanking: Tanking is a tactic where you just take out as many enemies as possible. Tankers would just head straight towards the biggest group of enemies and kill them. To be a good tanker you need no sense of self preservation. Your mind frame has to be "if i kill them then they cant kill me"
The best gun for tanking would be the M60 since it has a 100 bullet clip and a high damage rate.
Tanking is again useful in the same type of maps as rushing but with less corners.
(this is posted for the people who cant read pink people like me.)