A Parallel study is a type of clinical study where two groups of treatments, A and B, are given so that one group receives only A while another group receives only B. This is unlike a crossover study

Crossover study

A crossover study is a longitudinal study in which subjects receive a sequence of different treatments . While crossover studies can be observational studies, many important crossover studies are controlled experiments, which are discussed in this article...

where at first one group receives treatment A and later followed by treatment B while the other group receives treatment B followed by treatment A.