Portals allows you to place Workers remotely nearby the portal.
You place a portal somewhere and later, when you are somewhere else, you may still place workers on the fountains around the portal.
The number of portals you may place, their range and how long they last there, depend on the upgrades of your Portal Gateway
The color of the portal follows your Primary Color but not influence what workers that can be remotely placed.
You can not place Transmitters using a portal.