A holotape. A tape on which you can write your own message or upload existing ones to your Pip-Boy.
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.
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 (as of June 2012) 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.
Holotapes from the Deliver Message for Elmer quest require you to write text in both entry fields.
NOTE: As of June 2012, there is a known bug/problem when pasting text into a holotape. It has been reported on the 2238 game forum.