Room Acoustics:
Room acoustics play a significant part in shaping the sound that arrives at your ears. In addition to the direct sound from the monitors there will inevitably be some reflected sound bounced off walls, ceilings and any other surfaces in your studio. Because these arrive slightly delayed (having travelled further) they will cause phase cancellations / additions, affecting the tonal balance of the sound you hear. In the worst cases (think small box rooms with shiny wooden floors) the reflections may be almost as loud as the direct sound creating a very confused sound (like listening to your mix through a reverb plugin).
All rooms (and objects) will also have a what is called a resonant frequency, the frequency at which they will start to ring, a bit like a tuning fork. If your room exhibits an obvious ring then, again this can affect the accuracy of your monitoring. In the same way as you can identify frequencies to cut when EQing, set up a narrow EQ boost, turn up your speakers and sweep the EQ up and down, you'll probably hear that certain frequencies jump out.
Full room treatments can run into tens of thousands of pounds, but there are plenty of solutions that you can implement on the cheap: Move your speakers away from walls to prevent any early reflections - particularly out of corners which will cause a noticeable bass boost. In an ideal world the distance between your speakers the nearest surface would be at least twice the distance between you and the speakers. If you have a shiny wood floor then a rug will make a noticeable difference with the added benefit of being cosy under foot and try to break up any other flat surfaces with furniture (a sofa or bookcase along the back wall is a good option).
With the home studio revolution the cost of actual acoustic treatment has hit the floor so for just over £100 Universal Acoustics offer a kit that includes 20 acoustic tiles and 2 bass traps. With tiles placed directly above, beside and behind the listening position a kit like this is a very cost-effective way to get a decent sounding room up to scratch.
If you're going to get serious about treatment then it's worth spending some time working out the flaws of your room and the designing a solution to suit that need, using a variety of tiles and traps to absorb and diffuse different frequencies.