Cortana is a personal assistant for Windows Phone 8.1 users. Similar to Siri and Google Now, Cortana uses voice commands to interact with the user.
Standard commands of searching the Web, calling a contact, and sending a message are all possible through the service. But perhaps the most intriguing feature Cortana adds is the ability to set a reminder based on interaction with a contact.
In other words, you can ask Cortana to remind you to ask your brother about the date he had last week, to tell a friend a funny joke, or ask a coworker about the status on a big contract.
Setting a contact reminder is possible using two different methods. The easiest is to launch Cortana and say something along the lines of "remind me to ask my brother about his date." If you've already taught Cortana who your family members are, you'll be prompted to confirm the reminder and contact. If not, you'll be asked to clarify who your brother is, and then confirm the reminder.
The other method is to launch the Remind Me app and type in the reminder, and when it's time to select the trigger, choose the "Person" option. Then select the contact you'd like to act as the trigger.
The next time you talk to the contact, be it a phone call or text message, Cortana will remind you what you had asked it to. This is one feature Cortana has over iOS and Android, and one I find extremely useful. Oftentimes, actions we take aren't triggered only by time or location, but by interacting with someone.