I think it is Cortana.
In the US it is much more useful than for me in the UK, but it is still a useful tool. Firstly it is an easy way to use voice commands, but even if you don't want to speak to your phone or PC it's a good quick way of searching/doing stuff.
The more you put in the more useful it becomes, so in your favourite places put in your work, local shops, home, maybe your library, any place you go on a regular basis, then you can ask Cortana to remind you of things, for example "Next time I am at the library, remind me to ask if they have the book "David Copperfield", Cortana should then be able to remind you of this when you get close to your library as long as you have allowed GPS.
There are many other things it can do as long as you take the time to learn along with Cortana.