In SC:C, Sam has basically retired, however events of a massive plot bring him out of his retirement and force him back into saving the world once again. Splinter Cell has always been a series full of twists and turns so I won't go deeper into the plot if you don't know it. One thing I will mention is the interesting way of displaying objectives. To make it more immersive, similar to Dead Space, the objectives are shown on the walls, which like Dead Space does make it cooler than them just popping up on your screen as in Gears of War and most other games.
In SC:C, once you get close to an enemy you can press B to do a hand-to-hand execution. This grants you access to the mark and execute ability. When looking at an enemy you can mark them by pressing LB, and this puts a little arrow above their head, which is useful for tracking the locations of enemies. Once marked you can press the Execute button, which will make Sam shoot all the marked enemies he can see in one slow-mo sequence. Different guns have different amount of marks, but this can be a very useful ability.
As with the other Splinter Cell games you can often find a sneaky route around enemies, like up a pipe or through a window and so on, and it's still useful to turn off or shoot out lights so enemies cannot see you. However, in SP:C you will practically live in the dark. When in a shadow the world turns grayscale but with interactive/explosive objects like chandeliers and fire extinguishers as well as enemies still showing up in full colour.
Another aspect added in SC:C is the Last Known Position, which is basically that if you are spotted and get into cover then a white outline shows your last known position, and the enemy will head towards and attack that position, allowing you to sneak away.
In SC:C the weapons are the usual SMG's, Assault Rifles, Shotguns and the like, however what most of us tend to find ourselves using the pistol, specifically the five seven. This is for two reasons, first you have infinite ammo with pistols and second the five seven has the most marks available in the game.
Like most games these days, SC:C offers some specific challenges to aim for throughout your gameplay that grant you with upgrade points that you can use for weapon, armour and equipment upgrades.
Another really interesting addition is the Sonar Goggles, which when activated show you enemies through the walls, and you can even mark them using the goggles! You can also still use an optic cable to look under doors and can mark enemies using that as well.
In SC:C there is a fair amount you can do with a friend or online. There is an entirely separate Co-op campaign, unlike most games where you would just play the same campaign but have two or more of you, as in Halo. It's interesting as obviously the Co-op campaign adds more storyline.
There is also multiplayer which can be done split screen or over Xbox Live, with four main gametypes. There is Hunter where you have to clear the map of enemies, and if spotted instead of the ten enemies in the section you get a further ten added. In Infiltration it's exactly the same as Hunter but if spotted or detected you just fail. Face-Off is a gametype where you and the other player are pitted against each other, but with optional bots running around killing you both! Finally Last Stand is another Horde like game mode, where you have to defend an EMP generator from waves of enemies.
Another big thing added to SC:C, probably to highlight how Sam is acting on his own now and is a bit more brutal about it, is the interrogations that occur throughout the game. Basically you have a hostage with information and you can interact with the environment around you to torture them for information. It's all a bit messy, but very satisfying after spending so much time sneaking around.
Splinter Cell: Conviction has a lot to offer, both the single player and co-op storylines are quite interesting and fun to play through. The wealthy of gadgets and weapons, coupled with the level layouts, always provide a number of different ways to do a level. The multiplayer game modes are well worth a go as well, particularly hunter. All in all, another good game that is well worth a play, however I would recommend the stealthy approach as going in all guns blazing is a quick way to get you killed.