A ret2libc (return to libc, or return to the C library) attack is one in which the attacker does not require any shellcode to take control of a target, vulnerable process. So attackers use this technique a lot.
Every time you write a C program, you use one or the other of the inbuilt functions, like printf, scanf, puts etc. Have you wondered where the definitions of these functions lie? All the standard C functions have been compiled into a single file, named the standard C library or the libc. A libc is native to the system that you are working on and is independent of the binary (compiled program). You can use the ldd command to find out which libc is being used by an application.
$ ldd ./ret2libc
linux-gate.so.1 (0xf7fc7000)
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xf7d73000)
/lib/ld-linux.so.2 (0xf7fc9000)