A bug is a defect or error in a computer program that results in unexpected and unintended results.
The results of a bug can vary widely from impactful (e.g., a crash, a spin, or corrupt data) or subtle (e.g., invalid output, UI inconsistency, rarely encountered program state).
A bug is generally more than a simple syntax error that can be caught immediately by your development environment (IDE), but what can be caught by an IDE or compiler varies widely by language. For example, ...
A statically typed languages (C, Java, Swift, etc.) will highlight any attempt to assign a string to a floating point variable, while a dynamically typed language (Python, Ruby, JavaScript, etc.) will allow it.
Object C will allow you to send a method to a nil object pointer and not complain. Swift requires you to specifically indicate that you know the variable could be nil.
Bugs can always creep into code, but you're more likely to create or find them in code that...
Was written over a long period of time
Is larger and more complex
Was written by less experienced programmers
Was written by a larger number of programmers
Was written in higher level languages with fewer safeguards
Was written without using more defensive programming techniques (readable code, code reviews, writing tests, etc.)