The goal of epidemiology is to control or stop epidemics, by collecting data and conducting studies in order to make conclusions about how and why the epidemics occur.
To be exact, an epidemic is the occurrence of more cases of disease than would normally be expected in a specific place or group of people over a given period of time (www.cdc.gov). It is assumed that there is a "baseline", or normal rate at which a specific disease occurs in a population. Basically, an epidemic refers to when the rate at which the disease occurs exceeds this baseline rate.
A basic definition of epidemiology would be "the study of disease in populations." There are different aspects of epidemiology, each dealing with answering different questions about an epidemic, but they are all vital parts in reaching the common goal of stopping epidemics.
Keeping this goal in mind, the first aspect of epidemiology would be to look at the general distribution of disease (descriptive epidemiology)- dealing with basic questions like who and where the disease affects, and when it happens. Studying the distribution of the epidemic will usually reveal patterns and relations between certain factors.
These patterns are looked at closer using analytic epidemiology. The analytic aspect of this science involves using epidemiological studies to find the cause of the disease. Studies are used to test hypotheses about how and why the disease occurs.
The final aspect of epidemiology involves actually going out and physically solving the problem. Applied epidemiology is when epidemiologists use what they have learned (in analytical/descriptive epidemiology) to go out into the field to control or prevent the disease-accomplishing their original goal of stopping the epidemic.
Okasha, Mona. "“Epidemiology- Who Cares?”." Student BMJ (2001).