Basics

Stealth

The very first Splinter Cell was bred to be a game of stealth, but things have changed since then. Still, Sam's primary weapon of choice is the element of surprise. Stalking his enemies in the shadows, Sam loves to keep enemies in the dark (pun intended) and incapacitate them in these conditions. Any dark, shadowy area turns the screen black and white, which supersedes the need for any sort of stealth meter or radar. As long as Sam is veiled by this colorless drape, he remains invisible to enemies. Likewise, if the screen is completely "normal" he is vulnerable to enemy detection, at which point a great big Warning beacon lights up the screen. Once Sam is discovered, the beacon turns red. At that point, it is highly advised to become a ghost as quickly as possible. A silhouette of his last known position automatically materializes, allowing you some time to either flank the enemy or migrate to a safer area.

Executes

Executes are a testament to Sam's seasoned spy training and his unreal marksmanship. Whenever Sam takes down enemy using close combat (Deaths From Above, pulling guys out of windows, etc.), he gains an Execute ability. This allows him to mark up to a certain number of targets (as allotted by the currently equipped weapon) and instantly assassinate them with wicked precision. Any obstructions or a huge distance between him and the target may cancel the Execute effect, as indicated by a gray mark. Make sure all enemies you intend to Execute are called out with a clear red marker. Different weapons allow varying number of Execute targets at once.

PEC Points

Splinter Cell: Conviction has its own built-in points accruement system, wherein you ultimately earn an Achievement by conquering all the PEC challenges. A total of 38,000 PEC points can be obtained. These can be viewed on the menu screen and used as currency to purchase weapon and gadget upgrades, as well as costumes and their upgrades.

Game Modes

The game has three tiers of difficulty, which are Rookie, Normal, and Realistic. These modes present variable challenges by changing the number of enemies present at one time in an area and Sam's overall "health." Obviously, during Realistic mode, Sam is able to take only a few direct gunshots before its game over.

There are also other modes in addition to the solo storyline:

Co-op: Follow Archer and Kestrel, two Splinter Cells, as they go about their own missions. Play split-screen or online with a friend.
Hunter: Kill all enemies with conditions that you set yourself.
Last Stand: Defend an EMP device against enemies.
Face-off: Versus mode between Archer and Kestrel with the option to include more bad guys.
Deniable Ops: The single player version of Hunter and Last Stand.

Stay Crouched

Whenever possible (and especially around enemies), travel in a low, crouched position to minimize your chances of ever getting caught. Plus, when Sam is running around, guards will become suspicious of the thunderous footfalls that he causes.