Since Zerg and Toss players love complaining about how OP MMM supposedly is, I'd like to share some tips with you all, my fellow Terrans, on how to do it better =D
MMM means Marine/Marauder/Medivac, a combination of units that kills everything ever. Marines deal heavy damage to air and ground, Marauders slow units and soak up damage, and Medivacs provide healing and mobility via pickups and drops. Beware those who believe Marauders are the damage dealers of the bunch--against armored targets one marauder still does less damage than two marines, which are of equivalent in mineral price to that Marauder and don't cost any gas. The Marauder's real advantage is in health, where it has a solid 125, plus a starting armor of value of 1. They also have Concussive Shells, which slow down enemy units and make kiting possible (and when combined with Stim, easycakes).
MMM is a very strong strategy in every matchup at lower levels of play, and remains useful (especially in TvP) even at very high levels of skill.
The big easy strategy that works for pretty much everything is a one-base early stim push, which I tend to do like so:
-10 Supply Depot
-13 Refinery and scout with an SCV
-15 Orbital Command
-When the Barracks finishes, build 1 Marine, then get a tech lab.
-When tech lab finishes, get one marauder and start researching Stim when you can.
-Keep adding on Barracks until you have four, all the while constantly producing Marines and Marauders. Get upgrades from the tech labs when you have the gas (Stim Packs first, then Concussive Shells, then Combat Shields if you really want). You probably want 3 Barracks with tech labs, one with a reactor. Feel free to change that up though.
I usually roll out right and attack when Stim finishes researching, or when it's about to finish so it pops right when I engage the other player. Try to have around 3:1 Marine to Marauder ratio, but mess around with that amount if you think you need to. Do what feels powerful! If you want to try to break someone's ramp with this build (sometimes good, sometimes not) it is usually a good idea to poke up the ramp with one unit, or scan the top with your Orbital Command, to get an idea what's up there first. If you think you can break their defense, Stim and move up the ramp 'til you have sight of the enemy units, then attack!
After this first push is done, a good timing for a second push is when you get your first few Medivacs, which you should work toward after you've got the upgrades you want from the tech labs, as well as +1 attack from the Engineering Bay. Get a factory, then a Starport, and throw a reactor on it if you feel sassy. This is a great push to expand during =D Once you do, throw on two or three more Barracks to keep up your production.
Believe it or not, there is micro involved in MMM! Kiting is a great skill to have, and with Stim and Concussive Shells, it's never been easier. Pop Stim on your army, attack move toward the enemy, then run back after they fire a volley. Rinse and repeat, pulling back when they try to chase you and firing off a volley every time you get far enough away with your units. With practice many melee targets won't be able to even touch you anymore, ever. Especially Zealots.
Learn when to move and when to attack move; and especially when to target fire! This takes experience, but good bets are:
-Move close and target fire dangerous units like Siege Tanks or Collosi if you can. It's almost always better to have a specialized dangerous-unit-buster for such situations--like a Banshee or Viking, respectively--but that's not always possible.
-Don't move up ramps if you can avoid it, it compresses your army, gives your opponent a great firing arc, and loses firing time. Use Medivacs to spot if you can, or scans if you need to.
-Fight in open spaces! Attack move pretty much does the rest if your units have enough room to maneuver, and they will form themselves into a nice arc if you let them.
Also, if you believe Banelings are a Possibility, or anything else nasty to groups of weak units (like Collosi D=) position your Marauders up front before moving out. They are meatshields, use them as such!
MMM nearly always needs some helpers to be totally optimal. Consider these complimentary units:
-In Terran vs Protoss, Ghosts with EMP bust up shields like nobody's business, and they do nice damage vs Light Armor-type units besides.
-Also in TvP, but sometimes applicable elsewhere; get Vikings for annoying units like Collosi, especially if they have the range upgrade (which makes MMM cry). No extra tech required here, so do it.
-Banshees, if you can get away with them there's never a reason NOT to get Banshees. Great harassment, insane damage per second, and can force your opponent to get detectors if you research Cloak, which slows them down. You won't be able to get away with it often though, if you're against a good player =P
-Siege Tanks wreck armored and clustered targets, if you can throw down a tech lab on that Factory and pump a couple of these they will sometimes make your day much easier.
Tips, Tricks, and Extra Dealies:
-Don't be too afraid of cannons, they go down fast with Stim
-If you have the gas income for it, don't be afraid to get double Engineering Bays and pump infantry upgrades. Attack has priority over armor, but it's all dynamite.
-Select your units and right click the Medivac to load it up, not vice versa. It's much faster.
-If you really think you need to, it can be OK to get concussive shells before Stim. Do this with caution. Usually the reason for this will be lots of Stalkers, or *maybe* Roaches
-Get LOTS of Medivacs, as much as you can afford gas-wise. A slightly smaller army that is virtually un-killable is better than a bit larger army that is fragile. Also, having lots of Medivacs makes it easier to do drops, since you can bring more dudes.
Honestly, while MMM is a great starting build, and universally applicable, it's only really the best option you have in Terran vs Protoss. You can use MMM in Terran vs Zerg and Terran mirror, and it can be very effective if you're tricky enough, but generally 1/1/1 Mech builds are superior in both of those matchups. Don't worry though, Mech is OP too ;)