In the Apple Support Communities, Linc Davis gives an excellent answer to this question with a summary of the built-in security features of OS X 10.10 and the additional precautions you should take.
All versions of OS X since 10.6.7 have been able to detect known Mac malware in downloaded files, and to block insecure web plugins. This feature is transparent to the user. Internally Apple calls it “XProtect.”
Starting with OS X 10.7.5, there has been a second layer of built-in malware protection, designated “Gatekeeper” by Apple. By default, applications and Installer packages downloaded from the network will only run if they’re digitally signed by a developer with a certificate issued by Apple. Software certified in this way hasn’t necessarily been tested by Apple, but you can be reasonably sure that it hasn’t been modified by anyone other than the developer. His identity is known to Apple, so he could be held legally responsible if he distributed malware. That may not mean much if the developer lives in a country with a weak legal system (see below.)
Starting with OS X 10.8.3, a third layer of protection has been added: a “Malware Removal Tool” (MRT). MRT runs automatically in the background when you update the OS. It checks for, and removes, malware that may have evaded the other protections via a Java exploit (see below.) MRT also runs when you install or update the Apple-supplied Java runtime (but not the Oracle runtime.) Like XProtect, MRT is effective against known threats, but not against unknown ones. It notifies you if it finds malware, but otherwise there’s no user interface to MRT.
His answer is the third one down.