Everyone calls it something different - a Moleskine, a notepad, a diary. Some prefer an electronic form, some prefer pen-and-paper.

It’s more commonly known as… well a Commonplace Book:

A commonplace book is a central resource or depository for ideas, quotes, anecdotes, observations and information you come across during your life and didactic pursuits. The purpose of the book is to record and organise these gems for later use in your life, in your business, in your writing, speaking or whatever it is that you do.

From Marcus Aurelius to Bill Gates, commonplace books have a long and successful history, and I cannot recommend one enough.

Depending on how much structure you need you can start with just a Moleskine book like this one that you carry around with you all the time, or something more electronic like Sam here uses. Whatever works for you.