Environment lights simulate an infinite sphere light surrounding the scene. They are typically used along with a HDR probe texture which describes the color and intensity of the illumination for a given direction.
Environment light textured with an HDR probe
Portals are manually placed items that are used by the path tracer to better sample environment lights in complex lighting scenarios such as interior scenes.
For an interior scene, they are typically placed where openings such as windows are located. When placed correctly, render time savings can be substantial since the path tracer is aware about the location of the lights. For the same amount of samples and render time you get way less noise.
Typical scene configuration when to use portals
To use Portals, create portals using Create > Portal. Place and size them to properly cover window openings. Add them to the Environment Light Portals list and turn on the Enable Portals checkbox.