A microarray is a laboratory technique used to study gene expression. It involves attaching thousands of specific DNA fragments from different genes onto a solid surface, such as a glass slide or nylon membrane. The attached DNA fragments can then be used to measure the activity levels of each corresponding gene in a sample of tissue or cells by comparing their hybridization patterns (the degree to which they bind to their target sequences) to those of known controls. This allows researchers to quickly and accurately determine which genes are turned on, off, or expressed at different levels within a biological sample. Microarrays have many applications in fields such as medical research, agriculture, and environmental science, where understanding gene expression patterns can provide valuable insights into the functioning of organisms and their responses to various stimuli.