Mixing and mastering music is a complicated art. There are many factors you have to take into account and pay attention to. One thing that can be easily overlooked is the fact that different speakers produce sound differently. The way a track sounds on the headphones or speakers in your studio may not be the way it sounds if you play it in your car.
When you mix and master a track, you should try to listen to it on as many different speakers as possible. Play it on headphones, computer speakers, earbuds, cellphone speakers, car speakers, and any other speakers which the track might be played on. You don't want your track to sound spot-on in the studio, only to release it and then later discover it sounds terrible on car speakers.
Big name, professional producers often produce separate masters of the same track for specific methods of playback. They will make a radio master which is specifically mastered to sound best on the radio. A CD master is optimized for CD players. Internet mastering is for tracks being uploaded to the internet. You don't have to go this far with it, but you should always test a new track on as many speakers as possible before you release it. Think about where and how your listeners are most likely to listen to your track.
When in doubt, the best thing to do is play it safe. Take it easy on the bass especially. Bass-boosting a track should only be done if you are experienced with audio engineering. A bass-boosted track that sounds good on headphones might be an unintelligible roar on car speaker. The primary release track should be balanced across the board.
Remember, whether you're the artist or just the engineer, once your name is on something, it's on there for life. Listening to a track on a bunch of different speakers may seem like a lot of extra work, but it's well worth it in the end. You'll be able to release your track knowing that it regardless of how your fans listen to it, it is going to sound great!