In 757/767 there are three different kind of climb modes; CLB, CLB1 and CLB2. (Climb, Climb 1 and Climb 2 respectively)
CLB: This mode sets the N1 bug at upper EICAS (Engine Indicator and Crew Alerting System) page on the N1 indicator to the maximum N1 setting for the climb. This is the highest N1 setting that can be used for climb and and if EECs (Electric Engine Control) are turned on (they need to be in order to use the TRP (Thrust Rating Panel (this is where you set those TO, CLB, 1, 2, CON, CRZ modes))) they will automatically limit the power to this setting even if you pushed the thrust levers all the way to forward.
CLB1: This mode is exactly the same as CLB mode but the N1 setting is smaller, thus calling it 'de-rated climb' 1. With this mode you have less power in use and can save fuel.
CLB2: This is the third mode. Exactly the same as modes CLB and CLB1, but the N1 setting is smaller than in CLB1. This mode saves a lot of fuel during climb.
The way these modes are used:
For example, your climb will be done by LNAV driven by the FMC. The FMC has calculated your ECON CLB SPD as 308kts. This means, you get the most out of the climb ans save the most fuel by maintaining 308kts during climb. Now, you choose CLB mode after departure. By doing that, you limit the available power to the one specified by the CLB N1 setting in the EICAS. Now, you engage the LNAV mode in MCP (Mode Control Panel (the place where you can control AFDS (like autopilot))) and then engage the AFDS. Now, the FMC will maintain this CLB power setting and a vertical speed high enough to maintain 308kts.
Of course if you use CLB1 or CLB2 mode you have less power to use for climb and thus the FMC need to maintain a smaller vertical speed to maintain 308kts, thus calling it a 'de-rated takeoff'.
Hope this cleared something...it's 2am here, and getting pretty tired so I take all responsibility for all mistakes
1740 days idle. Beat that.