Finally explaining why I'm always at the back of the pack in online shooters (really, I'm sure), AnandTech's Derek Wilson has posted a very thorough dissection of how input lag occurs in PC gaming.
Input lag is the period of time between the input made by the user (typically movement of the mouse or a keyboard command) and when that input manifests in the game. For some users, it can mean a very sluggish gaming experience; for others, it may simply create a borderline imperceptible but nonetheless real disadvantage against players online who have more tightly tuned systems.
Unfortunately one of the key lag contributors seems to be the monitor, so if you're a stickler for visual fidelity and are using a *VA or IPS panel, you're likely going to have a tougher time than the users with the ultra-low-response TN panels, depending on how well the game is optimized for input lag. Of course, this is reductive of the massive amount of data in the article, so you'll want to check it out.