DA,
It's an RTS game. RTS games generally follow a rock/paper/scissors model.
Each player is given a few opening resources and a spot on the map. Then you gather resources and build units. The more efficiently you do that the more and higher level units you can produce. Some RTS games incorporate a hero type character that you can only have one of but that is much more powerful than the normal units. Generally, you will be able to pick from a group of heroes. Some games let you have more than one unique hero.
Starcraft doesn't have heros but it does have expensive units that are very powerful.
Then the combat is the rock/paper/scissors part. Broadly speaking, it works as follows.
Imagine these three units...
A grunt that has high melee damage.
An air unit that attacks from range.
A ranged unit that hits from far but is weak.
The Rock - melle unit loses to air unit.
The Paper - air unit loses to the ranged unit.
The Scissors - ranged unit loses to melle unit.
Hero and other powerful units can be micromanaged to attack too. Buffing type units provide area of effect +buffs and -debuffs.
As you can see, scouting your opponent is very important because if you know what he is building then you know what to build to counter.
There are different strategies to win but ultimately the goal is to kill all the opponent's units.
Rushing is generally a popular strategy online since it results in quick wins. A failed rush is a disaster.
APM - refers to Actions Per Minute. Almost anything you do counts towards your APM and it's a good indicator for how well you are micromanaging your units.
Good players will generally have a high APM and bad players a low APM.