How is a bug different from a beetle?

Strictly and formally, bugs and beetles are two different kinds of insects.

• A true bug (Hemiptera, meaning half wing) has a snout or beak that can pierce and suck. Bugs suck sap from plants and blood from animals.
• A beetle (Coleoptera, meaning sheathed wing) has two hard wings at the front which protect a second pair of wings, or the rest of the insect's body.

Informally, the Word bug is used to describe arthropods (insects, Spiders and crustaceans), so informally a beetle is just a kind of bug.