To the layman a monitor is a computer screen, but for anyone involved with audio and music, monitors are high quality speakers built to give you the best representation of sound without adding anything extra. Studio monitors vary in expense from the studio budget to the home producer price tag. Whether you are an experienced mix engineer or just starting out, having a pair of studio monitors is a must.
Maybe you’re a headphones power user and you might be asking, why should you invest in a pair of monitors when you already have some nice studio headphones? Truth is, you should have both. When mixing on headphones you eliminate the effect the room’s acoustics have on the sound. The benefit to having both monitors and headphones is that you can A/B to each, hearing what might be audible on your headphones but completely gone on your speakers. It is harder to get a sense of your stereo-field on headphones; small panning tweaks might seem more accentuated comparatively to your speakers. It is good practice to listen to your mix on as many setups as possible to see how well it translates.
If you are new to the studio monitor speaker world there are a few things you'll want to know before you drop one months rent on some shiny new monitors. First is deciding whether you need near, mid, or far field monitors. If you're just starting out chances are you'll be buying nearfield speakers. Nearfield generally sit closely to you in an equilateral triangle with your head, they tend to be the best for monitoring mixes and smaller compared to far field. Midfield, as you might have already guessed sit further away than your nearfield. They can represent a standard home stereo system. Last, far field speakers sit even further from you and tend to be the largest speakers in the studio, usually for evaluating mixes. Farfield can reproduce the nuances of a mix more effectively than nearfield monitors due to their greater size and power. Unless you have a properly treated room and thousands of dollars to make your studio as acoustically sound as possible you'll want nearfield to eliminate the possibility of your room distorting the imaging of the mix.
Next would be deciding between powered or unpowered monitors. The difference being that powered speakers have a built in power supply and do not require a separate purchase of a preamp. Using a preamp with unpowered speakers helps to eliminate the possibility of noise leaking in from the power supply, but a well built powered monitor should have very little noise.
Monitors cover a wide range of frequencies often 50Hz~ to 24kHz~. Each monitor you look at will range in size and have a different specification and frequency response. In most, if not all cases they will not have great low end frequency response as this requires a larger speaker called a sub woofer. Sub woofers are dedicated to the lows and make up for the lack of low end response that monitors tend to have. While not a requirement, having a sub woofer with your monitors will help you gain control over your low end and take your mixes to the next level (electronic dance music producers, for example, are particularly keen on monitoring the low end of their mixes, thus for them a subwoofer might be a worthy investment).
It is important to keep in mind that not all studio monitors are created equal. As mentioned earlier, monitors differ in price. It is a good rule of thumb to do a little research before making a big purchase such as this. If you have the opportunity it’s strongly recommend that you go to your local pro audio retailer. Often they will have a room dedicated to studio monitors giving you the opportunity to hear before you buy. If the closest retailer is hours away, don’t worry! Read user reviews and ask for help and recommendations in the forums.