What is the difference between 850nm and 940nm IR LED?

What is an IR LED? 
IR LED is infrared lighting emitting a diode.

What is an IR LED used for?
We can see these IR LEDs used on remote controls, sending programmed signals to the TV sets.
Or on CCTV cameras as night illumination, When there's no visible light in the darkness, IR LED provides night vision for the CCTV.
What is the difference between 850nm IR LED and 940nm IR LED?
The 850 nm wavelength is standard for most applications, including CCTV.
The light source does produce a faint red glow at direct exposure, but otherwise, it is not visible to the naked eye. Most of the true Day/Night cameras with removable IR cut filter have great sensitivity to 850nm wavelength, that’s why they are so widely used for IR illumination.
The main advantage of the 940nm wavelength is that it is completely invisible.
The light source doesn’t emit any glow. This is imperative for law enforcement, military, traffic, and railroad applications where red light can be interpreted as a signal. However, only a handful of cameras are sensitive to 940nm wavelength. The illumination range is 30–40% shorter, compared to 850nm wavelength. While selecting the 940nm IR illuminator, be sure that you really need it, and will be able to use it.

What is a invisible IR camera?
Invisible IR cameras are typically being referred to when speaking about CCTV security cameras that use 940nm IR LEDs instead of the more traditional 850nm IR LEDs.

So, what do these numbers mean? The 940nm and 850nm reference refers to the frequency of light emitted (measured in nano-meters) by the LED lights of the camera. 940nm LEDs are invisible to the human eye. 850nm LEDs are almost invisible, except that a red glow can be seen at the location of the LED itself.

For night-time covert surveillance, this is not ideal because the red glow gives away the location of the camera. There are both indoor and outdoor camera models available on the market that use 940nm infrared.