In Star Trek: The Next Generation, Geordi is blind. But yet he has a visor that enables him to see.
How does Geordi's visor work?
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This is explained on Memory Alpha:
The VISOR detected electromagnetic signals across the entire EM spectrum between 1 Hz and 100,000 THz and transmitted those signals to the brain through neural implants in the temples of the individual via delta-compressed wavelengths. (TNG: "The Masterpiece Society") The result was a vastly different visual acuity, with VISOR-wearers able to see in the infrared and ultraviolet ranges and beyond.
This was also confirmed in TNG: The Mind's Eye
When Romulans kidnapped La Forge in 2367, they were able to use his existing neural implants to tap directly into his visual cortex and secretly brainwash him.
So, it basically acts as an enhanced bionic eye which he wears over the top of his eyes. This then connects to neural implants on the side of his head (picture below - notice the little red 'dots' on the side of his head) which transmit this visual information to his visual cortex!
The signals are transmitted via delta-compressed wavelengths:
The VISOR sensed signals across the entire electromagnetic spectrum, and transmitted them to the sensory centers of user's brain via delta-compressed wavelengths ("transmitting the electromagnetic spectrum electronically to the brain"). (TNG episode: "The Naked Now", TNG episode: "The Enemy", TNG video game: The Transinium Challenge)
As to what Geordi actually sees, here are a few pictures:
Ten Forward seen through Geordi La Forge's VISOR. Includes examples of the written Romulan language. (TNG: "The Mind's Eye")
Data as seen through Geordi La Forge's VISOR with the visual acuity transmitter. (TNG-R: "Heart of Glory")