Wait, isn't ESO a class-based game? Well, yes, but ZeniMax says that classes won't "chain you to any one playstyle." Each class has three skill lines, and each line has different skills that are "thematically related to the class." When you combine those three lines "with the many, many other lines available to every character, each member of a particular class can be wildly different from the next," the company claims.
ESO is also tying some of its skill lines into exploration activities. For example, some skill lines have to be discovered out in the wide world of Tamriel, and ZeniMax took inspiration from Skyrim's Dragon Words when it designed additional skill point rewards that are acquired by finding hidden Skyshards. Finally, the dev blog touches on skill morphing, so head the ESO website and give it a full read.