Sort of hard to explain it without being able to draw a pic and I'm too lazy to create a Visio drawing at the moment, so here it goes...
A domain tree is formed when a child domain is created and associated with a domain root (looks like an inverted tree). Domains within the tree must be named identical to their DNS domain name. This is contigous name space.
(1st level, root)
(2nd level, nodes of the root)
If you host multiple domains trees, each represented by a different domain space, you have a forrest. Each Tree in the forest must be represented by a contigous name space.
If you have one tree with a single domain it is still considered a forest. A non-contiguous named single domain is still considered a tree.
So... for sake of argument, lets say we have dfwstangs.com and dfwls1.com as trees in our forest.
dfwstangs.com and dfwls1.com are non contiguous and so they are seperate trees. dfwstangs.com is the root of its tree, dfwls1.com is the root of its tree. Lets further say dfwstangs.com was created first. It is therefore the root of the forest as well.