Mechanics
Units
A unit in a strategy game is the thing you move around. Might be soldiers, tanks, planes or pizza delivery trucks.
Tech Tree
A Tech Tree is a concept used to describe the how things like units unlock. One example is a RTS game where you need to build a blacksmith building to make a Workshop and that workshop makes it possible to build Siege engines.
Fog of War
This simulates the unknowns of the battlefield or the world it is progressively revealed it to the player. The world often starts unexplored so you need to visit it to show it on the map. Then you also need some form of units there to see if anything is changing.
Real-time versus turn-based
Strategy versus tactics
Construction
Using resources to build units or buildings of any kind.
Resources
Resources can be in the form of money, oil, steel or any other that fit the setting. The player need to gather these resources and then spend them to build units and buildings.
Reference