It is as simple as this:
Crossfire is a fast paced FPS, with graphics that are ok (by that I mean they are not HD, but, fine), It has a Counter Strike like gameplay... But with original gamemodes...
Examples:
Ghost mode : You have Ghosts and Police... The Police must kill the ghosts... The Ghosts must kill all cops or explode the bomb... Note that having this kinda OVERPOWERED technology, the ghosts only carry a knife... While the Cops carry normal guns...
Zombie: 2 types of Gameplay:
-Stationary Map: Where you keep defeating Waves and Waves of zombies on the same place untill the Boss comes...
-On the Go: Getting through room by room, wave after wave, untill you reach the Boss Room.
Mutation Mode: Humans kill Beasts, Beasts must infect Humans... Nuff said
Hero Mode: Mutation mode, but you have this Hero, who has an Overpowered Gun, which can easily kill all mutants... Note: On this mode... To permanently kill a mutant (untill the round ends) you must headshot... Or else the mutants will just keep respawning until the round ends...
Predator Mode: Hero Mode, but instead of only a Human Hero, you have both a human hero, and a mutant hero...
It is a really cool game... Although from region to region it has its diferences...
Should totally try, its a lot of fun... Oh, and the best thing of all... Unlike most famous games (i.e. Combat Arms) you can buy guns through GP, money you get in game... Most of the RP/ZP/Vcoin/whatever (payed points depend from region to region) are only re-skins with better accuracy or better rate of fire...
It is Free to Play, although its not Pay to Win...