Yes. A midi controller usually looks like a piano (usually - not all of them do). They are different from electronic pianos in that they don't actually have any sounds loaded onto them and exist only to control midi performances on your computer using soft synths. In short, they require a computer to play anything whereas electric pianos do not. Don't worry though, Logic has a whole bunch of soft synths as does Garageband. You can also buy countless others covering all genres from 3rd party companies such as Native Instruments, EastWest Quantum Leap, Garritan Libraries, Vienna Orchestral Libraries, etc. They all range in price from a couple hundred all the way up to $11,000 for the complete Vienna Symphonic Library. Adding soft synth sample libraries will also eat up a WHOLE lot of hard drive space so external drives will be required if you ever plan to add more. For example, Kontakt 5 from Native Instruments is about 35-40 Gigs in size. The complete Vienna Symphonic Library is almost a terabyte.

Before you jump into getting an audio interface, check and make sure that your studio monitors don't have any unbalanced inputs (they look like standard RCA jacks). If they don't then yes, you will need an audio interface which will have the balanced outputs (that look like 1/4" headphone jacks or guitar cable jacks) to connect to them.

If they do have the unbalanced RCA inputs, then go to radio shack and buy a mini jack to stereo RCA adapter for a few bucks. Then you can just plug that adapter into your headphone output on your computer, then just use any stereo RCA cable to run to your monitors.