A research is conducted to assess the nutritional status of the children in a particular district. For this purpose, 2 children are randomly selected from every school in the district. This type of sampling is known as:
Cluster sampling 
This method is used when the population forms natural groups or clusters, such as, villages, wards blocks or children of a school etc. First a sample of the clusters is selected and then all the units in each of the selected clusters are surveyed. This method is simpler and involves less time and cost, but gives a high standard error. 
Stratified Random Sampling 
A stratified random sample is obtained using the following procedure: 
i) The  population  to  be  sampled  is subdivided into groups known as strata, such that each group is homogeneous in its characteristic. 
ii) A simple random sample is then chosen from each stratum. This type of sampling is  used  when  the  population  is heterogeneous  with  regard  to  the characteristic understudy. For example, to determine the prevalence of DMF teeth in different age groups, the different age groups form the strata and a random sample is to be chosen.
Simple random
Systematic random
Stratified random
Cluster