There are two types of glow engines: The four-stroke and the two-stroke.
Two-stroke engines are the most used, mainly because they are simple made, light, easy to operate, easy to maintain, and are usually inexpensive. Two-stroke engines operate at a high RPM and therefore can be quite noisy without a good silencer.
Nevertheless, the four-stroke engines also enjoy some popularity, mainly because they produce a lower, more scale-like sound and consume less fuel. They have lower power/weight ratio and lower RPM, but provide more torque (use larger propellers) than theirs two-stroke counter-parts.