The keystroke ⌘+ (in other words command and shift and equals at the same time) will enlarge the text size in many OS X applications. If you’re using a web browser to read this, try typing it now – the text and images on the web page should get bigger.
As a general rule ⌘+ works many places where there are big blocks of text, but not for smaller amounts of text, menus, or any pictures. Try pressing it when you’re reading mail with Mail.app, using Safari, or using the Help Viewer and watch what expands and what doesn’t. If you are using TextEdit, Pages, or similar then only the text that you have selected will enlarge.
The keystroke ⌘- (in other words command and minus) will make the text smaller in all the same places that ⌘+ makes it larger, so if you overshoot or don’t like the effect you can go back. Also, you have to enlarge the text for every new window and after you restart a program – the effect doesn’t stick.
On the plus side, the command works on all OS X computers – there’s nothing to download or configure or set up. And command-plus for bigger and command-minus for smaller is easy to remember. Try it anywhere – the worst that can happen is nothing!