Character journal/log/book is a player notebook element, which can be written to and is typically used to store various game details(such as quests/tasks details, crafting recipes, notes, to do lists). Usually it contains a function to edit/create/delete notes/scroll/books, or limited amount of text fields.

This UI feature allows player to remain in game without switching to read/write in a text editor. Typical text editing tools are present.
Notes and books can be shared between characters or in some games even sent via in-game mail(as letters/scrolls/etc).