Tape
A holotape. A tape on which you can write your own message or upload existing ones to your Pip-Boy.
Obtaining Tapes
You can buy them at Followers of the Apocalypse's Library in Boneyard area, and sometimes find them for sale by regular NPC merchants. You can acquire 2 holotapes for free by doing the Deliver Message for Elmer quest in Necropolis.
Uses
To upload a holodisk message to your Pip-Boy, select "Hand" icon on tape in inventory. To read messages in your Pip-Boy, look in your Pip-Boy archives. To write your own message into a holotape, select "Skilldex" icon on holodisk in inventory and then choose "Science" from list.
Each holotape has a 2000-character limit. The editing interface does not scroll down though so it is difficult to write long messages in a tape directly through the in-game interface. For longer messages, players usually write their text in an external file, copy and then paste into the tape.
NOTE: In order to paste text into a tape, the character's chat bar must be closed.