What is trainer ? is it legal to use trainer ?

Trainer - is standalone program that modifies computer's RAM (random access memory) in order to change behavior of game. All changes that trainer makes are made in RAM, which makes it quite hard to detect, however there are tools (such as anti-cheat engines) that scan your computer's RAM for changes that should not be there (one of them being Valve's VAC - valve anti cheat that is planted into Steam platform)

Lot of trainers inject their own custom code into game's memory (place in RAM where game is loaded) and use that instead of original code.
Way it works, is that programmer (like us) re-writes the original code in game's memory (RAM) and tells it to read the code from different location (it is needed because you can't always overwrite game's memory that does not have space for custom code, so its needed to tell the code to jump into new location where custom code is located). In the new location, is custom written code that does what we want (gives you infinite ammo, health, super jump ..etc) and after that, we tell it to jump back to original code so it won't crash after executing custom code.

All changes are made in RAM (random access memory) and RAM clears itself once in a while and every time you shut down your computer, so changes are not permanent and will not change any of your files. If you want to reverse the changes, simply reboot the game.

However there are exceptions. In some games, like TDU2, there are certain hacks that change things that will be saved into your savegame (like open all roads on map or tune car to legal level 1-4, those changes will be saved into savegame and cannot be undone)

History - In the early years of computer gaming (80s and 90s) trainers were integrated into games by cracking teams (such as RELOADED and FAIRLIGHT nowdays), so when you launched the game, you had trainer showing up and asking if you want to have "cheat mode" enabled.

Trainers were made standalone because integrated trainers were detected too easily and it also requires a lot more work to integrate a trainer. 

Name comes from - "trainer". Trainer is meant to train you to play the game, if you are rookie and can't beat the game, then you need a trainer to train you to pass that point of game. 

Useability and Details - Trainers are usually meant for single player games and most of trainers are free, however on some unknown reasons (at least our staff is unable to figure this one out) free trainers have very annoying sounds and music integrated into them and they also have horrible interface that just makes you go crazy .. maybe this is why lot of little kids are so retarded nowdays :)

However trainers are not limited to single player in any way, we, the ultimate online hackers, provide various online trainers. Some of our trainers are specifically meant for online usage and are using Stealth methods in order to bypass all sort of anti-cheat engines and checks made by game.

We also do not use code injection - it is used by 99.9% of trainer makers, but we use completely different methods, which makes our stuff stealth and extremely hard if not impossible to detect.
Instead of injection our own code into game, we find the place in game's memory and change it directly, without injecting our code to make the changes.

Here's example: if you have infinite ammo trainer made by someone else, they write a code that replaces the function that changes ammo (like ecx+eax,2) and replace it with memory jump (jmp 020300), so it tells game to jump into new location, where they will write their own code that tells the game that if you shoot your weapon, then it should not decrease the ammo at all, but what we do, is we write a code that FINDS the ammo counter and then simply writes different number onto it.
So if you shoot in game, game decreases the ammo from 50 to 49, but then our trainer instantly writes 50 there again, without injection our own code. Trainer simply scans the RAM in a way it takes only split of a second and finds the right position and just writes the value, this is not called code injection, because we do not alter the original game code at all.
This kind of method is impossible to detect, unless game server keeps track of your every shot (if that happens, your ammo will still run out, even tho it shows 50 on screen, but won't be detected by the server and you won't get banned)

Concept project - I (Sethioz) have started a concept project with help from Luigi Auriemma, that should produce 100% undetectable trainers. Using the method mentioned, where i do not inject any custom code, and Luigi's custom tool + script, i am 99.99% sure that this "trainer" is impossible to detect by any sort of anti-cheat engine. This is not even a trainer really, in sense of speaking it is, but i don't call it a trainer anymore. it runs for split of a second and changes whatever needs to be changed, so for example you can fill your ammo by running the trainer just once, but it won't freeze the ammo counter. This is another thing that makes it nearly impossible to detect, it only runs for split of a second and then closes after writing new ammo amount (it can do anything, give you super weapon ..etc)

Is it legal to use trainers - This depends on game developers / company, but i have never heard of anyone having legal problems because of using a trainer.
I have heard that Blizzard entertainment have sued some of the websites (such as cheathappens and MPC forum), but websites are still up and running, so based on that i think nothing big happend. However they made blizzard game hacks private, so you must first log into their site in order to see the hacks. Blizzard simply told them to take it off of public and nothing else really.
Keep in mind, this is because they published trainers / hacks and did not use them.

Using a trainer / hack is different, worst thing that can happen to you is getting permanently banned from the game where you use trainer / hack.

Conclusion - it is 100% safe to use trainers online (offline is even safer than 100%, no risk of getting banned either). If you use trainers online, you risk with permanent ban, but it is unlikely if you follow our trainer usage notes (like in TDU2, if you use our trainer to race against others and you happen to race against moderator/admin, you will get banned or in Mass Effect 3, if you use ultimate editor trainer to upgrade your weapon beyond what it can be legally, then they run a records check and if they see you have something that is impossible without cheats, they will ban you, all that is mentioned in our trainer descriptions)