There are two broad types of prevalence statistics: point prevalence and period prevalence. In both cases, more details need to be specified in order to fully describe the statistic.
Point prevalence is the proportion of people who have a disease at a given point in time. In other words, the total number of people in a given population who have a disease at a specific point in time divided by the number of people in that population. To be complete, it should include a case definition (how "have a disease" is defined), a population definition, and the particular point in time used.
Period prevalence is the proportion of people who have or have at some point had a disease over a period of time. Here, instead of the total number with a disease at any one time, you include anyone who had the disease at any point during the period. Depending on the period you choose, the population may change, in which case, you would typically use the population at the midpoint of the period.
A specific (and common) period to use for this statistic is "lifetime", which changes things slightly. Lifetime prevalence is typically reported as the proportion of people (alive now) who have had the disease at any point in their lifetime. The period in this case is not an absolute time period, but the lifetime of each person in the population.
You can learn more about this in the CDC's online text Principles of Epidemiology in Public Health Practice. Lesson 3 covers prevalence statistics.