1. Simple Random Sampling
"A simple random sample is a sample in which every member of the population has an equal chance of being chosen."

2. Systematic Sampling
"In systematic sampling, every kth member of the population is chosen for the sample, with value of k being approximately N/n."  
(N: # in population; n: # of samples)

3. Cluster Sampling
"A cluster sample is a simple random sample of groups, or clusters, of a population.  Each member of the chosen clusters would be part of the final sample."

4. Stratified Sampling
"A stratified sample is obtained by dividing the population into mutually exclusive groups, or strata, and randomly sampling from each of these groups."

Source: Donnelly text