A cat flap is a small door, often cut through a larger door, that is just the right size for a cat to go through. The flap part of the cat flap is a small door, hinged at the top, so the cat can go through but wind and insects can't.
A door is a way of closing off the entrance to something else, like the entrance to a house or room. Doors can be made of wood, steel or glass. You can lock some doors so that no-one can go through them.
A grate is a piece of metal with holes in it that is used as a lid or some other covering over a larger hole. The main purpose of a grate is to let liquids through, but stop solids getting through. Grates are usually used over drains. Very large grates like the one in the picture are strong enough for cars to drive over, while still letting water drain away. You may also be interested in grating food.
A hatch is a small doorway, often leading into a ceiling or underfloor space, but can be found in walls too.
A lid is something that is used to keep a container of some sort closed. You see lids everywhere. Soda and soft drink bottles have lids, jars have lids, some boxes have lids, and even some furniture like trunks have lids.
A manhole is a hole just large enough for an adult man to fit through. Manholes are generally used to access sewers.
A porthole is a small, round window typically seen on ships. Any small, round window on a building or anything else is also given the name porthole. Portholes on ships may not always be able to be opened. When they can, they must have a watertight seal.
A trapdoor is a kind of door that is in a floor rather than in a wall. Trapdoors usually lead to basements or cellars.
A vent is an opening that lets air in or out of a building. Vents might let air flow through naturally, or have air pushed through them by a fan or air conditioner.