Call of Duty 4:Modern Warfare (better known as COD4) is the first game in the Call of Duty series which is based on modern warfare. You will see a lot of familiar weapons and names in this game - AK-47u, MP5, Carbine, SAS, Marines, etc.
In my humble opinion, COD4 has got to be one of the best first person shooter (FPS) around. The popular Infinity Ward has done a great job this time. The single player mode is amazing. You get to play as a Marine and also as a SAS member - which means there are 2 separate stories here. The single player also provides you with different style of gameplay modes - assault, infiltration (where you need to move slowly through the swamp area), assassination (sniper) and others. The sniper one is particularly engaging. You'll know once you've tried it out. The single player mode in COD4 is easily one of the best in the series so far. Unfortunately, it's a bit short. It could have been better if there were more single player contents.
As for online play, it's good. I've spent countless hours on the multiplayer online mode. There are few game modes - capture the flag, team deathmatch, hardcore team deathmatch (for the experts) and others. The developer also provides you with a career mode where you get to progress through the ranks from 1 to 55. Certain weapons and perks (skills) will be unlocked once you've reached a certain level. And you get to customize your character - weapons (G36C with red dot scope) and perks (deep impact?). However, the online gameplay might be a little bit frustrating if you're playing in a game hosted by someone far away (Australia connecting to Canada, maybe?).
Overall, this is one of the first games I would recommend to my friend who owns a PS3. It has a good story, good single player and multiplayer mode, good choices of weapon and customization, good sound and graphics (good frame rate for a FPS). It's just that good.