Before learning about the entities, take a look at the entity window which is opened by pressing N. Here you can set all the properties of an entity, for example light strength, door speed, in fact everything that you can set.
Here you see information about the entity such as what different properties you can add and what they do.
Here you can see the different properties assigned to the entity, the name of the property called Key is to the left, the Value to the right. The checkboxes highlighted in red color are also properties you can, these properties are called spawnflags. To add a property which isn't a spawnflag, type its Key and its Value in the boxes highlighted in blue color.