Your question is very fundamental, i suggest you to return to a basic statistics or experimental designs books. And when this is the case posting similar questions here is not always better than searching online or reading from wikipedia or other resources.
Stratified sampling techniques are generally used when the population is heterogeneous, or dissimilar, where certain homogeneous, or similar, sub-populations can be isolated (strata). Under these conditions, stratification generally produces more precise estimates of the population percents than estimates that would be found from a simple random sample. BY that i mean stratification may produce a smaller error of estimation than would be produced by a simple random sample of the same size. This result is particularly true if measurements within strata are very homogeneous.
Simple random sampling is most appropriate when the entire population from which the sample is taken is homogeneous.
And which to choose is depend on your goal from the study, your time, the degree of detials you have in your data and sometime study cost.
