Methyl, ethyl and propyl are all alkyl groups that can be attached to aromatic ring systems. They all have different chemical and physical properties and all affect the aromatic ring system differently.
Methyl groups are derived from methane. They are composed of one central carbon with three surrounding hydrogen atoms. The carbon attaches to the ring system.
Ethyl groups are derived from ethane. They are composed of two carbon atoms and five hydrogen atoms. Ethyl groups only contain single bonds, and the ring system attaches to the carbon in the group with fewer hydrogen atoms attached.
Propyl groups are derived from propane. They are composed of three carbon atoms and seven hydrogen atoms. Propyl groups can have two forms. The normal, or n-propyl group, has three carbon atoms in a row with seven hydrogen atoms attached. In n-propyl, the ring system attaches to the carbon at the end of the group. In isopropyl, there are still three carbon atoms and seven hydrogen atoms, but the middle carbon atom is attached to only one hydrogen atom instead of two or three. This leaves a lone pair of electrons on the carbon atom for bonding. The ring system attaches to the middle carbon.
Other alkyl groups include butyl groups with four carbon atoms, pentyl groups with five carbon atoms; and hexyl groups with six carbon atoms.