Since you're in CS:S I will explain it in contrast to that game (seriously, without making fun).
CS:S - you have a crosshair
INS - you do not have a pretend crosshair. You use mouse2 to bring up iron sights
CS:S - you buy your equipment
INS - you choose one of the slots available, some of which have a choice of 2 types of guns, others are set
CS:S - you plant the bomb or rescue the hostages
INS - you capture objectives, or kill the enemy until they run out of reinforcements (or time runs out)
CS:S - when you die you wait for the round to end
INS - when you die you wait to respawn (unless your side is out of waves). INS rounds are much longer than CS:S rounds.
CS:S - you can take a lot of hits to the body
INS - you get shot, you probably die
CS:S - you can crouch (and run?)
INS - you can crouch, go prone (crawl), and lean left and right, and run
CS:S - small maps
INS - some maps are quite big
CS:S - ...
INS - teamwork and communication is very important
CS:S - has a radar that shows the enemy if your teammates can see them
INS - has a map that kind of shows where your team is
That's all I can think of right now.