A holodisk or holotape is a high-density, laser-readable, manufactured-crystal computer data storage device produced by Wattz Electronics, based on Tape Holography technology, storing their information safely for extended periods of time. THT Tapes can store an incredible amount of data, some can hold as much as 256k. The standard tape holds 64k, with the middle model holding an average of 128k (barring any bad inches of tape). The most modern models can hold over 4,096 gigabytes (4 terabytes) of information and weighs 4 pounds.
It can be used either in stationary computers or in your PipBoy.
For a list of holodisks in each game please refer to the main articles linked below.