Modern rackets come in 2 head shapes, i.e. isometric and oval-head.
The tip of an isometric racket head is more squarish compared to an oval-head racket.  The illustration on the left shows an isometric racket whereas the one on the right is an oval-head racket. Notice that the tip of the isometric racket head is ‘flatter’ or more squarish, compared to oval-head racket.

An isometric rackets gives you a larger sweet spot, hence making it easier for you to hit a shot efficiently. An oval headed racket has a smaller sweet spot but if the sweet spot is hit, it gives you a slightly more concentrated power.

Isometric rackets technology has matured so much that most of the rackets nowadays are of isometric head type, except for rackets such as Yonex Carbonex series. Take note though that different isometric rackets might have different head size, i.e. some are wider and some are narrower (although the difference may not be too obvious). A larger racket head will widen the sweet spot relatively, but it adds weight to the racket though.

As a general rule, an isometric racket is easier to play with compared to an oval racket, as it gives you a larger sweet spot. Unless you have played consistently and efficiently with an oval-head racket, get yourself an isometric racket.