Fog of War
The Fog of War covers everything that is beyond the sight range of units or structures. On your map, you'll see dark areas and grayed-out areas. Dark areas have not yet been explored, while the grayed-out areas have (they simply are not within sight range of friendly units or structures). Although terrain and structures can be seen through grayed-out areas, enemy troop movements and newly built structures remain concealed.
Ramp control allows you to prevent enemy units from accessing an area (generally your main base), funneling them into a narrow corridor that will also serve as a killzone. The enemy will not be able to attack your units from below unless they gain vision, one of the reasons the high ground is a tactically advantageous position.
Smoke or Undergrowth
Units inside obstructing terrain can't be seen by units or structures on the outside.
A Xel'Naga tower that is 'controlled' (by having a player's units adjacent to it) grants vision over a good chunk of the map. It is recommended to use cheap units, like workers, marines, or zerglings, to control the tower. However, if you seek to deny your opponent of the tower's benefits (or if you just have to have it), send in a detachment or troops and add a few defensive structures.
A zergling uses the tower and undergrowth to watch for nearby enemy units.
Hellions use smoke and a Xel'Naga tower to their advantage.
Oftentimes, you'll find rocks standing between you and a critical location like an expansion, or a backdoor into an enemy's base. Rocks usually require time (or a large army) to destroy. If your base has a backdoor sealed off by destructible rocks, make sure you place a structure or unit nearby to keep watch for impending sneak attacks.
Rich Mineral Fields
Yellow mineral fields yield more resources than their blue counterparts, but you'll usually need to do something special to get to them: destroy rocks blocking the way, fly to an island, or set up shop in a very vulnerable location. [Learn more]
At the beginning of the game, a sneaky terran floats their command center over to an area with patches of rich minerals.
A zerg player sneaks an expansion at a resource spot with rich minerals.