All three of the Mass Effects are primarily an RPG, with a full voiced main protagonist. In conversations, there is a dialogue wheel in which you choose your character's response. The top option tends to be "Paragon" options, being Heroic, the middle in the Neutral option, staying calm, and the bottom is the "Renegade" option, being a Badass. On the left you will have "Investigation" options, which are pretty much asking a question to find out information.
The rest of the game is a third-person, cover shooter. The first has the weakest combat mechanics, but the following two had the combat system revamped, and is very fun. You choose from four(ME1)/six(ME2)/five(ME3) weapon classifications: Pistol, Assault Rifle, Shotgun, and Sniper Rifle(ME1-3), SMG(ME2-3), and Heavy Weapon(ME2, in ME3 they don't take up a weapon slot, but can be found on the battlefield). In ME1 the different weapons in a class, ie. a Banshee vs an Avenger rifle differ only in aesthetic and stats, while in ME2 the difference between weapons is in aesthetic and handling, while in ME3 the weapons differ in aesthetic, stats, and handling.
In addition to your weapon, you also have a variety of abilities. You can use a variety of tech attacks to disable opponents shields, weapons, abilities, or freeze, burn, or stun them, or (in the case of ME2-3) summon turrets and drone to aid you. Biotic powers allow you to throw your enemies around, rip them apart, stun them, shield yourself, or (in the case of ME2-3) charge into them instantly. Combat abilities allow you to take more damage, cancel out negative effects, or (in the case of ME2-3) add additional ammunition effects to your weapons.
You don't fight alone, as two members of your team will join you, and lend a hand during combat, as well as their opinions during missions. Speak with your crew aboard ship to learn more about them, and even being a romance. See how your actions and words affect not only your character, but your team, and the universe as a whole, utilizing Mass Effect's save port system to play the latter games with the choices you made in the previous ones taking effect, for better or for worse.
From what I've read, the trilogy also includes the DLC from ME1, the DLC(excluding weapons packs and character appearance packs) from ME2, but not the ME3 DLC. You will, however, have the Online Pass, allowing you to download all 5 or 6 multiplayer DLC's for free.
I'd definitely recommend it.