Acetyl is the radical of acetic acid. It contains a methyl group single-bonded to a carbonyl. The carbon of the carbonyl has an lone electron available, with which it forms a chemical bond to the remainder of the molecule. The chemical structure is

O=C-
  |
  CH3

The acetyl radical is a component of many organic compounds, including acetyl-CoA, the neurotransmitter acetylcholine, and the analgesics acetaminophen and acetylsalicylic acid (better known as aspirin).