You usually see autocorrect of typed words on smartphones but OSX Yosemite does the same thing too. It auto-corrects words that you don’t want to be autocorrected and the fix for that is easily. OSX Yosemite resembles too much iO7 and that has a lot of people complaining.
To disable autocorrect:
Go to System Preferences > Keyboard > “Text” tab
Then uncheck the “Correct spelling automatically” tab.