What is the difference between a bug and an insect?

Bugs are a specific type of insect. All bugs are insects, not all insects are bugs.

“All bugs are insects, but not all insects are bugs. Beetles are not true bugs, nor are butterflies or bees or flies.” Here

edit – clicked when no answer was there

Insects have three body sections, three pairs of legs, an exoskeleton, antennae… Uh… Some other requirements that I’m forgetting…

Bugs are gross.

Bugs have a specific kind of mouth and a specific membranous type of wing.

That’s “true bugs,” which are an order of insects. Most people don’t mean that when they say “bugs.” Informally, ‘bugs are gross’ and even things like slugs count.

Insects are a specific kind of animal (a class of arthropods) and insect is a scientific term. Bug can be used in a general sense to mean any small land arthropod (a “creepy-crawly” thing) that includes insects, spiders, and various other kinds of animals. In this sense bug is not a scientific term.

To an entomologist, however, a bug (more properly “true bug”, as pointed out by @dpworkin) is a specific order (hemiptera) of insects, used in a narrow scientific sense. To further complicate matters, many insects known informally by common names that include the word bug are not actually true bugs at all. For example, lightning bugs (aka fireflies) are a kind of beetle—neither bug nor fly.

So, depending on context, either some insects are bugs or some bugs are insects. Such is the nature of language.

I think bug is just a word for creepy crawly things, sort of like a slang. They have to be small, and can fly, too.
For people who don’t know that an insect requires a head, a thorax, an abdomen, two antennas, six legs and sometimes wings, it’s a bug. (IE spiders, centipedes, larvae, that weird thing that just crawled in my ear.)

I agree with @Symbeline and others who made the same point. “Bug” is an all-purpose word for any kind of little creepy thing, from spiders to beetles to centipedes. “Insect” is more specific. Spiders, centipedes, worms, millipedes, pill bugs, are not actually insects as they don’t meet the definition by way of their body type or number of legs.

I also think it’s a little strange that there is an group of insects called “true bugs” when “bugs” doesn’t always refer to insects. Who came up with that idea? What makes them more “true” than, say, a ladybug?

I think the scientific term bug had its popular (not scientific) definition widened by misuse and misunderstanding. It really bugs me! (couldn’t resist!)

@MissAusten The “true” bugs came first, and the other creatures were later mistakenly called bugs. No one “came up” with a confusing category of “true” bugs after the fact.

@dpworkin OK, I will accept that and try to be less irritated by it.