so im new to the warhammer 40k rp games but have been playing warhammer 40k for a long time and i was wondering what the diffrence betwen the diffrnt games is.

I mean, its the difference between an RPG and a strategy game.

In an RPG, you play one character. In a strategy game, you control an army.

The fluff is the same, just that the scale is completely different. Also, the rules are different, so be sure to actually read them. Knowledge of tabletop 40k doesn't really help you too much (knowledge of the fluff is good though).

no i mean th diffrnce bettwen dark heresy deathwatch and rogue trader

In DH you essentially play detectives of the 40k universe.

In RT you play Privateers trying to amass a fortune to dwarf that of any other.

And in DW you play Space Marines doing what they do best.

As illustrated by Control, each game catters to a different genre.

Dark Heresy: Mystery.

Rogue Trader: Adventure.

Death Watch: Action.

Technically, the rules are roughly the same across all three games, with a few variations on how certain combat actions/skills can be applied, and varying systems for psychic powers. Overall, its very easy to houserule in an overuling system (as easy as saying "Dark Heresy rulebook trumps all others," although with psykers, some work may be needed).

As others have said, they mostly cover the key themes of mystery, exploration/adventure, and action.

Mechanics wise, the characters have different "levels of power,"

Dark Heresy = 400 xp - 13000 xp range (higher with the ascension rules), characters are acolytes, ranging from adepts to guardsmen to psykers or arbites

Rogue Trader = 5000 xp - some number I don't remember (roughly middle of the ascension xp range though), characters are high nobility

Deathwatch = 13000 xp and beyond (to the same max ascension sets), characters are space marines

There is no built in notion of multiclassing, so characters made within one ruleset are generally built within that games rules (so Dark Heresy characters don't train in Deathwatch character classes, etc.).

Each game has fairly different economies. In Dark Heresy, you use "thrones," basically money, to buy things. In Rogue Trader, you apply your profit factor (a broader notion of how insanely rich you are), to acquire things. In Deathwatch, you build up renown to gain permission to use cool toys, and spend requisition per mission to have gear (and trade it in for the next mission, which is nice because it means you can try out all sorts of equipment, rather than be stuck with some expensive piece of gear).

Beyond that, the games are as fun as the GM and players help shape them to be.