Here's a summary of the differences between an option and an ETO:
Options can occur only when one season remains on the contract, while ETO's can occur when two seasons remain if the contract is for six seasons.
Options can be included in any multiyear contract, but ETO's are allowed only with five or six year contracts.
Options can be held by the player or the team, but ETOs are always held by the player (i.e., there's no such thing as a team Early Termination Option).
Option years may not have a lower salary than the previous season. ETO's have no such restriction.
A contract with a player option can be extended when the option is not exercised. A contract with an ETO may not be extended if the ETO is exercised.