Generally speaking, conversion rate is a percentage of how often a particular goal is achieved based on the number of times a specified action was taken to achieve it.
A conversion can be almost anything such as a form submitted online, a sale of an item or service, a phone call, a registration for an event, a completed survey, a click on a certain advertisement or banner, a visit to a specified web page, and so forth.
In order to determine conversion rate, systems of measurement have to be in place either online, offline or both. Goals and rules for measurement have to be worked out in advance to ensure results are accurately tracked and calculated.
For example, if one of the goals on your website is to have visitors fill in and submit your online form, then your conversation rate would be how often that occurred within a given time frame, say each month. To measure this goal it would be useful to implement an online tracking application such as Google Analytics to track the number of visitors to your website and the number of forms successfully submitted.
The formula to measure conversion rate for this particular goal would be:
number of forms submitted online ÷ number of visits/month = conversion rate %
12 ÷ 2,400 visits/month = 2%