You must have a mouse plugged in.
System Preferences > General, under the "Show Scroll bars" section, there are three options:
- Automatically based on mouse or trackpad
- When scrolling
Each act differently.
Automatically... with a mouse plugged in will show the scroll bars. When you are just on the trackpad, the scroll bars will be hidden until you scroll. This is the default setting.
When scrolling will always hide the scrollbars unless you are scrolling. If you are on a MacBook, this is probably what you are most used to seeing, so turn this on if you use a mouse occasionally and hate the look of the scroll bars. (This is how I have mine set).
Always will always show the scrollbars, so if you like them, they will always be there for you.
[Edit: I just saw that there was a response up at the top. Hopefully this here will help someone searching to find the answer quicker.]