what is a microarray?

It is a chip with thousands of DNA oligonucleotides spotted on it. Each spot contains a set number of oligonucleotides specific to something, like a gene.

The most common application is to take cDNA (which is made from RNA) labeled with certain dyes. These are then hybridized to the microarray. Since the microarray contains oligos specific for a gene, the cDNA from that gene will be complementary to the oligo and bind to it.

Since there are different levels of RNA for each gene in a cell, some spots will have higher levels of cDNA bind than others. Because the cDNA is labeled with a dye, this can be measured, giving a relative amount of how much cDNA is binding to each spot. This allows one to measure the expression level of thousands of genes simultaneously.

There are other applications as well, but I wont go into those.