A bug is nothing more than an error or mistake in either software programming or hardware design.
Bugs can manifest in any of several ways, from simple and benign misbehaviour to catastrophic failure, crashes, and data loss. In fact, many bugs many never manifest at all.
Bugs can also be exceptionally difficult to identify and correct – or trivially simple. A somewhat cynical rule of thumb is that the harder a bug is to identify, the easier it is to fix, and vice-versa.