Bug can mean many different things:

Biology

• True bug, a group of insects
• Bug, a slang word for a pathogen that makes organisms ill
• Bug, a slang word for an insect, such as an ant or a beetle

Technology

• Software bug, a problem with the code in a computer program
• Bug, a device for telephone tapping
• Bug, a nickname for a Volkswagen Beetle because of its shape

Other

• Western Bug and Southern Bug, two rivers in Poland and Ukraine
• Something that irritates somebody, such as "My loud neighbour really bugs me!"