Cover is any barrier that makes it more difficult to attack a target. This includes hard cover such as walls that prevent the attack striking the target, and visual obscurement such as bushes or smoke that make it harder to aim at the target.
Players do not need to state they are using cover to gain an advantage from it. A character is automatically assumed to be getting the best possible cover from any location.
It is recommended that cover is 'eyeballed', i.e. quickly assessed and a 1d6, 2d6 or no bonus awarded. This keeps the game moving quickly. However, sometimes it is a matter of life-or-death and players will want to work it out exactly.