In the System Preferences application, you’ll see an icon called “Language and Text”. If you open this, and select the Text tab, you’ll see a list titled “Symbol and Text Substitution”, which provides some useful text shortcuts. You can use these to auto-correct common typos as you make them, or to replace short text mnemonics with longer words or phrases.
A handful of shortcuts are provided built-in. One will automatically change (c) into a © symbol. Another changes “teh” into “the”, automatically correcting a very common typo.
Each one can be switched on and off using the checkbox next to it. They don’t work everywhere, particularly in third-party applications, but almost all Apple-made apps support these shortcuts just fine.
You can use the controls at the bottom of this list to add your own shortcuts. Click the plus sign to create a new snippet. Enter the thing you want to replace, with the thing you want it replaced with.
For example, if there’s a URL you often type, or an address or email boilerplate you use all the time, just devise a simple abbreviation you can remember, but which won’t crop up during normal typing.
This built-in system is convenient, but pretty basic. If you need text replacement shortcuts with more flexibility, investigate third-party paid-for options like TextExpander