Very easy question on ulimit

ulimit looks just easy to use but... "-c sets the maximum size of core files created" what is a core file? "-l sets the maximum size that may be locked into memory" What is it? "-m - the maximum resident set size" What is it? "-s the maximum stack size" Is this the maximum stack size of a single program, right? "-t maximum amount of cpu time in seconds" In seconds? Seconds in a minute, or what else? Thanks in advance, Tazio