What makes a C standard library function dangerous, and what is the alternative?

Question

While learning C I regularly come across resources which recommend that some functions (e.g. gets()) are never to be used, because they are either difficult or

Answer 1

To add something about strncpy most people here forgot to mention. strncpy can result in performance problems as it clears the buffer to the length given.

char buff[1000];
strncpy(buff, "1", sizeof buff);

will copy 1 char and overwrite 999 bytes with 0

Another reason why I prefer strlcpy (I know strlcpy is a BSDism but it is so easy to implement that there's no excuse to not use it).