Level, Class Level, Character Level, Caster Level, ECL (Effective Character Level), ECL (Effective Caster Level), ECL (Effective Class Level)
Level – This is a general reference and is entirely dependent upon context for meaning, generally refer back to the most recent specific reference. As an example, if someone is talking about a Fighter and makes a vague reference to level (like, 'each time you level') the context would be Fighter Class level.
Character Level – they is the total of all your class levels, no adjustments. If you have a character that is Druid Level 5/Wizard Level 4/Rogue Level 6, you have a Character level of 15. Character Level is a simple total of levels. Usages for this number would include things like the Druid feat Shape Focus which allows you to use your character total levels when determining your level for shape shifting.
Effective Character Level (ECL) – This is a number used mostly by your GM/DM. It is your Character Level plus all adjustments like Bonus Dice, Racial adjustments, other factors that would affect the difficulty rating of your character. Your GM/DM will use this number averaged with the rest of your party to determine appropriate challenges, dungeon levels and experience gains for encounters.
Class Level – This is the starting level for the class plus each level you have added to that class from experience gains. This is used to determine BAB, SAVES, and specials gained for that class.
Effective Class Level (ECL) – This can get more complicated, because it can work as an across the board effect or as a partial effect for class level benefits. Effects, items, game devices that alter the class level up or down are usually specific as to what they affect. An example would be a spell that boosted your BAB and granted you saves and effectiveness as if you were the appropriate level for the new BAB, this would increase your Effective Class Level accordingly. Effective Class Level normally affects non-casters where Effective Caster Level will apply to casters.
Caster Level – This is Class Level applied to casters. In addition to BAB, SAVES, and Specials, it will include base level Spells Known and Spells available.
Effective Caster Level (ECL) – This can be more complicated than Effective Class Level for non-casters. The same basic rules apply, the biggest difference is determining where and how they apply. Most items that grant a caster ECL unless they specify otherwise apply after determining Spells Known and Spells Available and simply adjust the caster level power at which you cast, changing SR, DC on Saves, the power level of the spell. Other Effective Caster Levels like those granted from a PRC are added before determining Spells Known and available so you can learn higher level spells and cast more spells as if you were at that adjusted caster level. Yet some Effective Caster Level effects will increase the power level and number of spells without granting you the ability to know higher level spells. You have to read the descriptions. One caster can potentially need to deal with multiple ECL effects at the same time. Perhaps the PRC he has grants him a +2 ECL, a feat grants him +1 ECL for spells per day, and a ring grants him +2 ECL for power level, this character gets +2 ECL for level of spells known, +3 ECL for spells available, and a +4 ECL for Power and DC for spells that he casts.
Yes, ECL is used in different places with a different meaning, just like the uses of 'level' you have to read it in context.