Having the right gear ratios on your bike will make a huge difference to your riding experience. Here's a quick guide to how they work.
Your gear ratios are defined by the size of your chainrings and the ratios offered by your cassette. Your tyre width also affects this to a certain extent, but by far the biggest influence us exerted by the chainrings and cassette.
On a road bike you have two choices of chainset. Standard, which is commonly 53x39 and compact, commonly 50x34. Cassettes generally offer gears in the range of 11-25 or 11-28.