Close, but not quite.

Area portals are a VITAL part of mapmaking; correctly placed area portals on a correctly designed map can allow for overall high-poly and high-detail environments with little framerate drops.

Normal area portals slice the BSP into sections; if one of these sections aren't visible at all through environmental geometry (meaning scenery/devices/bipeds/vehicles not included), it is not rendered. If you do not define area portals or define them incorrectly, what isn't visible will still be rendered, and thus hog up processor time.

EXACT area portals (a.k.a. exactportals) are portals you put into spots like windowframes, doorways, cave openings, etc. If a polygon is not even partailly visible through an exactportal, it is not rendered.

Exactportals must seal off any openings; a good, basic example is this: let's say you model a map with a house, complete with windows and doors. You must put an exactportal in EVERY WINDOWFRAME OR DOORWAY THAT LEADS OUTSIDE, NO EXCEPTIONS. If you miss a spot, it will not work.

One more little tidbit: you usually shouldn't put normal portals inside of an area sealed off by exactportals; if you look into the area through the exactportal, happen to keep looking through a normal portal inside, and then hit another exactportal on the other side of the area, no environmental geometry will be rendered past that second exactportal. Here's a crappy ASCII diagram to better explain what I'm talking about.
Code:
O ->     []             |             []                 NOTHING OVER HERE
^Player   ^Exactportal  ^Portal       ^Exactportal       WILL BE RENDERED
The HEK Tutorial goes over creating basic portals, but I feel portalling is hard to explain; as you make maps and experiement, you'll pick up on it.