There are many forms adjectives can take - one is the participial form. This is also known as the -ed or -ing adjective. These words are adjectives with an ed/ing ending that express an idea of giving and receiving an action.
-ING Adjectives are the doers of the action:
This is a very boring book. (The book creates the boredom.)
The amusing comedian made me laugh. (The comedian was amusing.)
The horrifying movie made me scream. (The movie was horrifying.)
-ED Adjectives are the receivers of the action:
The bored audience were angry. (The audience was bored by someone.)
The amused man laughed all night. (The man was amused by someone.)
The horrified people screamed. (The people were horrified by something.)
You could also say that the -ing adjectives have an active meaning and the -ed adjectives have a passive meaning. (One does the action and the other receives it.)