What is a cresol? and what are the different types of cresol?

It's a common name for a methylphenol or hydroxytoluene, take your pick.

There are only three cresols: o-cresol, m-cresol, and p-cresol, due to the symmetric nature of benzene. Their densities are similar. They each have a hydroxyl group and a methyl group.

These are ortho, meta, and para isomers, which indicate 1,2, 1,3, and 1,4 substitutions, respectively.

They can be used as precursors to synthesize things like bromocresol green (a pH indicator, green at ##"pH" = 3.8~5.4##), butylated hydroxytoluene (an antioxidant), and so on.