shift + S
Your default tool. Used to select vertices, edges, faces, meshes, objects, and groups.
Translate Tool (move)
Move the selected object in different directions. You can use the arrows to drag the object to a different location.
Rotate the selected object on any axis. The rotation is snapped to a defined angle. You can change the angle on the bottom right.
You can also press and hold the ctrl key and drag to rotate without any snapping.
Scale up or down objects. Also works for props.
Change the center pivot point of an object. Can be useful to translate for example a 90 degree tunnel. This is a temporary change, unless you use 'Set Origin To Pivot'.