When I'm out and about, I need a shopping list or information about my errands. I used to carry a small notepad so I wouldn't forget but now I can use my phone instead because I carry it with me wherever I go. Both Google and Outlook.com include a note feature in your account that can be used in place of the paper notepad. I like being able to make notes on my desktop computer and then have it automatically appear on my phone.
Microsoft's version is called OneNote and it syncs Outlook.com with the Windows 10 app as well as the Android app. I use it regularly for creating shopping lists, meeting notes, and reminders. OneNote notes can be organized in notebooks and each notebook can be organized into sections (tabs) and each section can be further organized into pages. Each page can have multiple floating note elements.
Google Keep is Google's note app and it is more of a sticky notes than a notebook with 4 elements: notes, lists, voice notes, and photos. The notes are arranged in the app like a bulletin board without any hierarchy other than the newest notes are at the top. To organize your notes, you can pin them (so it stays on top), change the colour, and give the note a label. Through a Chrome extension, you can assign categories to note colors. Keep has no text formatting other than adding check boxes to convert lines of text into a list but it allows you so set time-based or location-based reminders for your notes. Time-based reminders appear in Google Calendar, in both online and Android while location-based reminders vibrate your phone when your GPS indicates you're near.. You can also take voice notes that Keep will transcribe into text. You can also copy your note to a Google doc. Like other Google features, can access Keep via the Android app or online through a browser.
Many people use Evernote, which allows you to work across various platforms but the free version allows syncing only across 2 devices (eg, decktop & mobile phone).