DNA methyltransferase is an enzyme whose function is to catalyze the transfer of methyl groups between molecules, in this case, DNA.
There are two main types of methyltransferases, de novo and maintenance.
De Novo methyltransferases recognize something in the DNA that allows them to methylate cytosines de novo. These are expressed mainly in early embryo development and they set up the patten of methylation.
Maintenance methyltransferases add methylation to DNA when one strand is already methylated. These work throughout the life of the organism to maintain the methylation pattern that had been established by the de novo methyltransferases