This is a lot of info about VAC for people who don't know a lot about VAC!
If you already know enough, that's all good just click away.
VAC = Valve Anti-Cheat
1. VAC is a software that is built into several games on steam, VAC works a anti-cheat... (Obviously)
2. VAC has been around roughly since 2002
3. VAC will try to connect to your computer and check for running services and processes.
4. Your computer will then reply back with the information of you game that you are running, this will show all of your files and such that are used to run the game... Along with any other files that you have added yourself. 
5. When VAC receives all of you information of your running processes and such it will compare your files with any detected or any unrecognized files, if any of your files are match that database of detected hacks and such your steam account will be VAC banned over time.
6. If VAC does not recognize a file that you have that is not detected and is not known, it will be tested and if the file is confirmed as something that would give you some sort of advantage or anything of the sort that modifies the game in a way that is unfair the file will be added to the database of detected files / hacks!
7. When your account has been VAC banned you will not be able to join any VAC secure servers which means for any Counter Strike, Team Fortress and so on, you will be kicked when you join a VAC server, which is most of them...
8. Another event that will happen when your account has been VAC banned is it shows on your profile how many VAC bans you have and how long ago you received them.
9. When you receive a VAC ban all of your in game items in your account inventory is locked to that certain account so you cannot trade it with a alt account and continue to hack with the items.
10. One of the most sad things about a VAC ban is the fact that it is permanent. Whatever you do, if it be contact steam support, the VAC ban will stay.
11. When you are VAC banned you are not just banned from the one game you were hacking on
* Team Fortress 2
* Day of Defeat: Source
* Half-Life 2: Deathmatch
* Counter-Strike: Source
* Condition Zero
* Ricochet
* Day of Defeat
* Team Fortress Classic
* Half-Life: Deathmatch
* Deathmatch Classic
12. When you get a VAC ban you can no longer Trade, Sell, or gift any of your in game items.
13. You will also never receive item drops on VAC supported games.
14. When you are hacking a game you can not just assume since you did not get banned right away that you are safe and in the clear... No, we wish, VAC bans can take anywhere from 6 hours to 6 days, to 6 weeks to even months and months, it all just really depends.
15. The only way to really fully avoid a VAC ban is to not modify the game content / files. If you modify or add any .exe or .DLL to the game, you will most likely be VAC banned depending on what the file is.
16. Just remember VAC is not the only thing you have to worry about if hacking the game, for example CS:GO has overwatch and such, you get the point don't be dumb with the hacks or you will get banned by overwatch also.
17. My advice is to use a alternative account while hacking, or modding... Since VAC only bans accounts when you get banned it will not be your main and you will just lose a account that has nothing on it!
Thanks and have fun and try not to get caught!