Ok, let me explain things because people apparently have waay to much stuff confused.
CR Does Not EquL ECL.
If CR=ECL, then a Young Red Dragon could be a 6th level character. Obviously, This Is Not Balanced.
The CR system is a little wacked to begin with. Let's look at a level 8 fighter. He has a CR 8. This means that two level eight fighters wailing on eachother is much more a challange, then the CR system is set up for. No. CR means Challange Rating, as compared to A PARTY. Yes. A PARTY. A CR is how hard something is, compared to 4 PC's. Thus, a Troll, a CR4, is an Average Challange for a 4th level party.
An ECL (Equivilent CHaracter Level) is how you balance a Single Character. How you balance someone to be in a group of equal characters.
Let's look at your standard, assumed party. One fighter, one wizard, one cleric, one rogue. Let's set the party's level at... Six.
Now, you can have all four characters at six level, of assorted race, by the PHB. Watch, as we add ECL's. Let's use the Tieflling, ECL +1
The fighter is level six. However, if he were a Tiefling fighter level six, he would be considered Level Seven. Thus, having a level seven character in with a sixth level group is the same. HOWEVER, the tiefling were 5th level, then he would be considered a Sixth Level Character, to fit into the party.
Werewolves have an ECL of +3. Thus, if one of the characters were a True Lycanthrope (Afflicted don't count, because that's just a disease), a 3rd level character would be considered 6th level, to enter the party, and be considered the same power and equasion with them. Why is it done like this?
Werewolves, for instance, can change shape. They get a +2 str, +4 dex, and extra feats. Not only that, but skill bonuses. THis is figured up into three level equivilents. THey sacrifice three levels, to get the benefits of the class.
Same with tieflings, who get a +2 to str, and a -2 to Cha, but they can cast Darkness, and have energy resistance 5 to fire, and two other things. Not only that, but they are Native Outsiders, thus Hold Person, Charm Person, Dominate Person, etc, do not work on them. This is considered the equivilent of sacrificing a level, to gain these powers.
One more time, I'll give another crack.
CR is based on an equasion of four (The basic party size).
ECL is figured on an equasion of 1, because it is considered a PC. Why? Because some of the powers that a creature can have, may come into play with a PC only once. Spell Like abilities that work once a day, or at will, make little difference in a fight, but when a PC can use them at will, it's a *big* difference.
