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Good evenig everyone, a friend of mine just showed me a ps3 he bought called apparently (Jailbrek and cex) Well, he told me you can play emulate games and all sort of stuff

Could someone explain what it is please ? Is this like a custom firmware of PSP ?

Yea pretty much.

CEX is the term for a retal PS3 and DEX is debug.
You can convert retails into debugs but its not 100% since come beta builds don't run for some reason.

But yes it sounds like he got a PS3 with a CFW already on it.