A bug is an insect. You might refuse to go camping because of your intense dislike for bugs. Bug can also be a verb meaning "annoy." Most likely, bugs bug you.
There are bugs you can see, like bees, and bugs that are much smaller, like viruses and bacteria. If you get one of these microscopic bugs, it means you're sick. You can also say, casually, "I've caught the model train bug — I'm hooked!" A microphone that's hidden in someone's home or telephone is also a bug, named after its small size. To bug someone means either to spy on them, or simply to bother them relentlessly.