A Bug is an error within the software, which malfunctions while running. And because of this Bug, error flashes, causes a fault in functioning, or does not behave the way it's intended or designed and thus producing unexpected or incorrect output or result. A Software that contains many Bugs, or produces too many errors could be termed as Buggy or defective.
If you wish to get trained and certified in Software Testing and wish to break into the testing domain, check out the Software testing online course from Intellipaat that offers you with 20hrs of instructor-led course and 40hrs of guided projects to help you enhance your practical skills. If you are a beginner and wish to get started right away, I recommend you watch the following video on Software Testing Methodologies.