runtime
runtime is the time when your compiled code gets executed . runtime errors - errors which were not detected at compilation time but appeared during run of compiled code.
runtime libraries - your compiled code needs other codes to execute . Let say if you use printf() function in your program , compiler should add the binary of the printf() to your program binary at compile time (static library linking) or put reference to that function within binary (dynamic library linking) - symbol pointing function to be called.
Some runtime errors could arise from improper usage of C such as poor casting from one type to another (mentoined by The Svin ) , buffer overflows when buffer bounds are not checked before access it, improper pointer usage ...
It is recommended to use lint or similar code consistency checker tool o your source code. It will provide much more warning than your favorite compiler . Also there are safer libraries which replace your compiler's to avoid flaws in your program.
There is a book "linkers and loaders" ...