-ing

The suffix -ing has a few different functions in English.

Examples
Here are some examples:

Continuous Verb

 * am running

Noun

 * watching: Here 'watch' is a verb, and the -ing adds the grammatical category ASPECT to the verb. This helps the listener know that the action is still not completed yet. i.e. The action is still in progress.

Adjective

 * interesting face: Here 'interest' is a noun and by adding -ing, the word class of the word changes to an adjective.
 * high-paying job
 * exciting movie
 * boring textbook