Select allows selecting several objects at once. Objects that are selected are placed in the Selection object, which permits iteration. Selecting an object (say, a column) activates the object.
Activating an object makes it the active object.
Best way to think of it is “many cells can be selected, but only one may be the active cell at any given time.”
Note: They have one thing in common: they are rarely needed and can slow down your code. You can usually work on an object without selecting or activating it.
Best practice is not to use either unless necessary.