H=hydrogen
R=alkyl group
X=let's say a halogen
A free radical reaction is composed of three steps.
In the first, initiation, a free radical is produced by some process (exposure to light, heat, etc). So, it the creation of a free radical requires energy in some form.
The general equation for this reaction is:
X:X ----> (energy input) X. + X.
This means that instead of one of the elements taking both electrons and one losing both, as is typically the case in organic reactions, each species gets one electron. Free radicals are highly reactive because they want to complete their octet (kind of like ions, but not charged).
Thus, the second step of a free radical reaction is for these reactive elements to find another molecule to react with. This step is called propagation. It requires less energy than the first step because the free radicals have already been created. Essentially, the free radical cause a bond to be broken in a molecule. One of the elements in that molecules forms a new bond with the radical, itself creating an extended radical, while the remainder of the molecule is also a radical:
X. + H:R---> H:X +R.
As long as a free radical reacts with another non-radical species, then propagation continues ad infinitum and the molecule grows in each step.
R.+C=C---->R-C-C.
R-C-C.+C=C--->R-C-C-C-C.
However, if two radicals react with each other, this is called termination, the final step in the chain. The reaction is terminated because two radicals combine with each other, leaving no radicals to further react; the reactive species are consumed.
H.+H.--->H-H
H.+X.---->H-X
X.+X.----> X-X
This is sort of the opposite of the initation step.
These reactions are useful in alkane addition reactions, alkyl halide generation.
Unlike Sn2 reactions and like E1/Sn1, reactions with more substituted species are favored. I.e., the formation of tertiary radicals is favored over secondary, etc.
As far as entropy and enthalpy... enthalpy is essentially the energy required or produced in a reaction. All else being equal (T and S), reactions with a negative enthalpy are energetically favorable while those with a positive enthalpy are energetically unfavorable (that is, some kind of energy must be put in for the reaction to occur). Enthalpy is probably the most significant factor (as far as organic is considered) in the Gibbs free energy equation:
G=H-TS
So, for all intents and purposes, when H is negative, your reaction is favored and vice versa. In terms of free radical chain reactions, initiation has a positive enthalpy while propagation/termination have negative enthalpies).
Also, reactions where entropy is positive are favored. As far as I can recall, entropy is not as important generally for understanding organic reactions as enthalpy.
The other stuff, I don't remember.
