The Portal Key is the most important item in Ingress if you want to create links and control fields. A Portal key can be acquired in a couple of ways: hacking portals or destroying links and then picking it up (Portal keys drop some of the time when you destroy links and they drop at the originating portal for that link, not the terminating/destination portal).
Each Key is specific to one portal, but you can have multiple keys of the same portal. When hacking a portal in hopes of collecting a portal key, your odds of acquiring one through a hack are far greater when you do not currently have that key in your inventory. If you already have a key to a portal and you are needing to farm multiple for an operation, your best bet is to drop the key on the ground, or to place the key you are farming into a capsule.
In your Items tab in the OPS menu, you can filter your items to show only Portal Keys. They are sorted by two different methods. Either distance from your current position (closest ones are first in the list), Or sorting by name. On the portal key you can see its level, faction, current percentage of XM, Xm mods being used on the portal, a photo of the portal, portal name and address, and the distance of the portal from your current location, as well as the Nav button. The Nav button will direct you on the distance you are from the portal you are currently set to navigate to. Once selected you can go into a portal to see it’s mod’s resonator energy levels, and if it’s owned by your faction and you have a key or multiple keys to it or to drop keys. From this view you also have the ability to remote recharge the portal (only if you have a key to the portal).
With new updates, when viewing keys through the inventory menu. you can now easily see how many keys you have for each portal, this can also be seen if you click on a portal and have multiple keys
There is a lot more to learn about Portal keys and how you can use them: