A drain is a type of hole, usually attached to a drainpipe, where water can escape from somewhere like a bath, washbasin or road. The entrance to a drain is usually covered by a grate.
You can use the word drain to describe the process of the water leaving somewhere, or the hole through which the water escapes.
A fissure is a relatively deep, narrow hole in something. Some fissures are natural features of things, others are caused by external damage.
A gap is a small space between two things, or a small break in something continuous like a fence. A gap can be measured in distance, like the gap between the two buildings in the picture, or it can be measured in time. For example, some people take a gap year between school and university.
A hole is where a piece of something is missing. The door in the picture has two holes in it.
