The Twilight Forest realm is an endless world like regular Minecraft. Nearly all of the Twilight Forest is densely forested. It has more of an enchanted, fairy tale, and mythological theme and feel than the main Minecraft world. The skies are perpetually dim, giving a darker, somewhat gloomy cast to the world below. An overstory of larger trees further shades most of the world below, creating a canopy covering most of the world (so thick that you can actually walk on it most places.) The canopy is pierced only occasionally by massive trees that rise to the ceiling of the world. The terrain is flatter, or at least less mountainous than regular Minecraft, but you will occasionally encounter hills, sometimes rising far above the canopy level. These hills are hollow too, containing caves filled with valuable ores, treasure and dangerous monsters. You may also run across many different dungeons, bosses, structures, trees, and a numerous amount of unique items throughout your travels through the Twilight Forest realm.
You can make the portal by making a 2x2 pool of water and surrounding it with flowers, plants, mushrooms, etc. Then you throw a diamond into it but stay back because lightning will strike the portal. Finally it will turn purple like the nether portal and you can enter. I recommend marking your portal in the twilight forest because you can easily lose it even with the minimap.
Finished Portal
For more information go to http://twilightforest.wikispaces.com/
(entering the portal can sometimes crash servers but not all the time)