The Terran Marine

Against The Zerg

Weak Against : Baneling ,Brood Lord,Ultralisk

Strong Against : Zergling 

OK Against : Hydralisk,Roach,Mutalisk

Against The Protoss

Weak Against : Colossus, High Templar, Archon, Zealot

Strong Against : Immortal,Phoenix,Void Ray

OK Against : Carrier Dark Templar, Mothership, Sentry, Stalker

Against The Terran 

Weak Against :  Hellion, Siege Tank

Strong Against :  Thor

OK Against : Banshee, Battlecruiser, Ghost, Marauder, Reaper,Viking


Marines are the basic combat unit for the Terran race.They are cheap,fast to produce and can atack both air and ground units.
A good Terran player will never use marines on their own as they get killed pretty fast.The best role for the marine is that of support.Squads of marines are always ideal in combination with other units especially hellions and marauders.



Upgrades

• Stimpacks : Researched at the Baracks's Techlab,this upgrade increases your marine's movement and atack speed for 15 seconds at the price of the marine's health .Altough everytime you use the stimpack ability and your marine looses 10 HP,it nearly doubles the damage output of the unit.The extra speed increase is especially valuable for hit and run tactics,kiting or a fast escape.

• Combat Shield : Researched in the same place as the Stimpack,this upgrade adds 10HP to the marine.Recomended only if you plan to go with a heavy marine army.

Using Marines Against The Protoss 

Marines are always a good support unit against Protoss players,but they cannot survive alone against them.Early in the game they are easily killed by Zealots,but they can fend off a Zealot atack if they are mixed with Marauders.Marines are a strong counter to Void Rays,a squad of upgraded marines can easily destroy an early Void Ray rush.


Using Marines Against The Zerg

Marines are your first line of defense against that pesky zerg rush.They excell at killing zerglings and can be quite effective against Hydralisks .
They are especially dangerous against Zerg players when they get mixed with Marauders and Medivacs,the popular MMM strategy being very hard to counter as a Zerg player.
They are also very good at countering Mutalisk rushes,but fail hard against banelings.Its always important to avoid bioball armies if you see your oponent with a Baneling nest.

Using Marines Against The Terran

Marines are not a great option when playing against other Terrans.They can be good against Banshees but Vikings are a better option at that stage in the game.You can use them as cheap damage in any army formation,or as a meat shield against against certain units.