Services are context specific so only services that apply to the object selected are shown. Any app can add tools to the Services menu, so your list of Services might well be different from that of another Mac user.
Here are some of the basic Services that all users should have and how they work:
- Speech / Start speaking text. Select any amount of text and your Mac will read it to you aloud.
- Mail / Send To. Select an email address in any text and a new Mail message will be created to that address.
- Mail / Send Selection. Select any amount of text and a new Mail message will be created with the selected text inserted into the body of the message.
- Look Up in Dictionary. Select a single word and the Dictionary application will open with the definition of that word.
- Search With Google. Select a word or phrase and a Google search will be performed in Safari.