Autocorrect seems to be enabled by default in OSX 10.11 El Capitan which can be a pain especially in Mail, Skype and TweetDeck or any application which you are typing a lot and simply don’t need it.
Put a stop to It Globally
Go to Apple Menu > System Preferences >
Click on Keyboard > Text and turn Autocorrect off by unchecking Correct spelling automatically
Turn off AutoCorrect per Application
Or if you see the benefits of it for most Applications and just want to disable one or two – then you can do this in the Applications Edit Menu under Spelling and Grammer and disable ‘Check Spelling While Typing’
This also works for OSX 10.10 Yosemite and OSX 10.9 Mavericks.