HEAD SIZE - The head size of a racket is probably one of the easier to spot differences in a racket. Head sizes tend to be between 90sq" and 105sq". You can follow the rules that the smaller the head size the more control you will have, but the harder it is to make a clean contact. The larger your head size the more the strings will act like a trampoline, so you get increased power but less control.
Tom's Opinion - 95-100sq" rackets tend to suit most people. If you want a bit more spin, go for the 100sq".
WEIGHT - Rackets tend to weigh between 270g and 340g. If you go light (270g-300g) the more manoeuvrable the racket is, but you sacrifice stability on the tougher, harder shots. If you go heavier (300g-340g) you allow the racket to have more mass to swing through the ball, as long as you can start the movement and achieve the correct timing.
Tom's Opinion - Play with the heaviest racket you can manage for the longest time you ever play. So for example - if you play with a 320g racket but your arm gets tired after the first set of a 3 set match, it is too heavy for you. You want to play the first shot just like the last, so use a racket that you can manage.
BALANCE - Simply, it is where majority of the weight is situated in the racket. More professional players tend to like a head light balance as it allows you to keep manoeuvrability on a heavier racket. Perfect to hit big from the baseline but to move the racket quickly up at the net to volley.
Head heavy rackets allow the hoop to have more mass hitting the ball, but they tend to be lighter in weight. Some experts believe that this is a cause for a lot of case of tennis elbow as there is increase vibration in the handle.