Bug means an error in the program or in the system, because of which the software application gives unexpected behaviour. Most software errors occur because of mistakes made by the developers in its source code, or in its design.
Some errors occur due to incorrect work of the developer tools, for example, because of the compiler that generates the incorrect code. A program that contains a large number of errors severely limiting its performance is called unstable or, in the slang language, "buggy".
The term "bug" is usually used to indicate errors that manifest themselves at the stage of the software operation, in contrast, for example, from design errors or syntactic errors. The report containing the information about the bug is also called the bug report. The report on the critical error is called the crash report.
Software bugs are localized and eliminated during testing and debugging of the program.
Depending on the stage of software development, on which the bug is detected, there are:
Syntactic errors (recognized by the compiler and making compilation impossible) - for example, the absence or inconsistency of the opening and closing parentheses;
Warnings - for example, using an uninitialized variable. In this case, the compiler may notice that the programmer is doing something unusual (probably wrong), and reports this, but the programmer decides whether to ignore the message or not;
Runtime errors, semantic errors - for example subtracting variables instead of adding or segmentation errors.
Depending on the nature of the error, program and runtime, the bug can occur immediately or vice versa - to remain unnoticed for a long time.
Also, the bug can be manifested in the form of a vulnerability that makes possible unauthorized access to the system. Program testing at each stage is a necessary part of any software development process.