The established keyword is only applicable to TCP access list entries to match TCP segments that have the ACK and/or RST control bit set (regardless of the source and destination ports), which assumes that a TCP connection has already been established in one direction only. Let's see an example below:
Suppose you only want to allow the hosts inside your company to telnet to an outside server but not vice versa, you can simply use an "established" access-list like this:
access- list 100 permit tcp any any established
access-list 101 permit tcp any any eq telnet ! interface S0/0
ip access-group 100 in
ip access-group 101 out