Grenade Tactics (CS:S)
Grenades are one of the most important aspects of CS:S. One Flashbang can kill your whole team or save your whole team; it's as simple as that. I am going to try and give some handling tips for the Flashbang and HE grenade. For smoke grenades see Achtung's "A quick guide to smoke grenades".
As I just mentioned, the Flashbang is a very powerful tool in CS:S, but how do you use them? Well this is quite simple in theory; in summary: keep them unexpected and high. Protecting yourself from a Flashbang directly above you is almost impossible, even if you duck and look into a corner you still get flashed for a few seconds. Try it yourself, throw a Flashbang into the air, look at the ground and see the WSoD (White Screen of Death).
Another advantage of throwing Flashbangs high is that they are harder to spot, therefore the enemy has less time to react. There are some positions where it's not possible to get the Flashbang high into the air without making yourself vulnerable. In these cases you need to get the Flashbang just about round the corner when it goes off. This makes it almost impossible to avoid unless your enemy was looking away in the first place. To do this throw the flash at the floor about a meter away from where you want it to go and it will bounce around the corner.
Avoiding Flashbangs takes a lot of practice. It requires fast reactions and a lot of thinking. You may think it's as easy as turning away, but it is much harder than that if you want to reap all the benefits. Many times even identifying the grenade being thrown at you can be difficult. As Achtung mentions in his article for Smoke grenades, they can be used as fake Flashbangs. Smoke grenades look nearly identical to the Flashbang apart from the fact that they are primarily grey with no yellow stripes on them. This can be used to identify a decoy. Now, if you turn away from an HE grenade, it can be fatal as you get hit for a lot more damage in the back than if you remained facing the exploding grenade. Obviously HE grenades are far easier to spot than a Smoke grenade decoy. The He grenade is an entirely different shape, it's green and if that's not enough for you, it sounds a lot deeper and clunkier when hitting walls or objects than Smoke or Flash grenades.
Flashbangs are the most useful grenade for disorienting your opponent and can act as a decoy too. A single Flashbang can flash multiple defending opponents; giving you the upper hand as they will have no clue what's going on. They can't tell if you are rushing or not, so they will call for backup. At this point you can then rush another site, or pick off the rotators.
The HE Grenade
The HE grenade is also a useful tool, although not as useful as the disorienting Flashbang overall. The most important tactic implemented with He grenades is the "double nade". Two or more defending players both throw their grenades into a confined space where the enemy could be rushing (such as long A on de_dust2). Even if the HE grenades do not end up not right next to the enemy, as long as each grenade does 50 damage to the enemy player, then they will be dead because there are two or more grenades. This "double nade" tactic in Counter-Strike Source broadens the AOE (area of effect) by increasing the damage.
Remember, as I mentioned in the Flashbang section, you take less damage in the front with a HE grenade than the back, so if you see one coming in your direction, look at it and jump backwards. HE grenades are really one of the main reasons to buy body armor. Wearing body armor decreases the damage taken from grenades by quite a bit.