Determining which tire size you want is dependent upon what you are going to be using them for. Most road wheels have a diameter of 700c, though some models could be as small as 650c. The other number on a tire size relates to the width. If a tire is 700cc by 25 millimeters, then the width of the tire is 25mm.
The ideal width of your tire depends on if you are using it for racing, training or commuting. The skinnier the tire is, the faster it is going to be. A 700 x 21mm tire has less contact with the road and offers less frontal resistance to the wind. The trade-off is, skinny tires wear quicker and are more vulnerable to damage to the sidewall. They also require a higher air pressure, which results in a harsher and less comfortable ride.
For commuting or training, a wider tire is likely your best choice. They make balancing and cornering much easier, offer more contact with the road, are less likely to puncture on rough surfaces and will last longer. The downside is they are heavy, not as aerodynamic and can cause problems fitting into brake calipers or certain bike frames.
The bottom line is, no single bike tire is likely to fit the needs of every cyclist. It's important to research each brand of tire and find the one that works best for you.