If you often misspell words whilst typing then Google Docs spell check has a great feature that allows you to train it to recognise those words and auto-correct them as you type.
Most of us have developed annoying little habits over the years of typing, the classic one possibly typing "teh" instead of "the". Google Docs helps you combat these habits with the Always correct to.. feature, allowing you to select an automatic substitution in any Google Doc you create or edit.
There are 2 ways you can set up an auto-correct or automatic substitution:
Right click on the misspelled word and select Always correct to "x" - Google will suggest a suitable word
Click on Tools > Preferences. Here you can add new automatic substitutions or edit your existing corrections.
This feature is also really useful for adding shorthand replacements to your Google Doc or for adding symbols such as fractions (when you type 1/2) or copyright.
This is a brilliant feature that will get better as you teach Google some of your habits or shorthand, it's a shame it isn't available in Gmail yet, hopefully it's in the pipeline.