show users - show sessions

If you want to see who's logged onto a router, use command show users

BE-607NR01#show users
    Line       User       Host(s)              Idle       Location
*  6 vty 0     admin      idle                 00:00:00 10.32.20.1
   7 vty 1     admin      idle                 00:00:22 10.32.20.7

  Interface    User               Mode         Idle     Peer Address
  Vi3                             PPPoATM      00:00:00 80.1.2.2

This shows the open sessions to your router. The line with the * is the current session, where we used the command. If you want to kill an open Telnet session, use command clear line x

Reference:
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/5.x/command/reference/sh_ti_tr.html#wp1028456

To see the active sessions from your remote device, use command show sessions
If you enter a sessionnumber you'll access it, use disconnect to leave.
Ctrl-Shift-6-x brings you back, it also suspends Telnet/SSH sessions.

XX-001NR01#show sessions

% No connections open

Another useful command is show line which gives a nice overview.