Half-Life entities can be separated into two categories:
brush-based entities have a shape and size that is built, out of brushes (or 3d shapes), into the map by its creator. They may be visible, as in a func_door, or invisible, as in a trigger_multiple. You can easily recognize a brush based entity because they all have a "model" attribute, which specifies what brushes make up the entity.
point-based entities don't have any shape or size that a mapper defines. Because of this, they have an "origin" attribute instead of a "model". These include such things as lights, monsters, player spawns, and more.
If you want to learn about the different entities, I suggest practicing using Worldcraft, looking at the .fgd file, and reading the many half-life editing websites that are out there.
So, let's take a look at that .ent file and break some stuff!
Here are some rules of map hacking.
1. You may change/add/delete any attribute of any entity--including the type of entity(classname)--as long as it makes sense for the entity to have that value. The .fgd shows all the values for the various entities. I recommend using Worldcraft to look at the .fgd in a (somewhat) friendly format.
2. You may not remove any entities from the list, nor can you insert any or otherwise change the order. This may cause the map to not work correctly.
3. You may add new point-based entities onto the end of the file as you need them. Once you have made the changes you want to your map, use Ripent to import the entity list into the map, replacing the original one. Go back to page on for instructions on that.