ECC is a method to detect and fix errors, so with ECC ram you use more bits per byte, i think 2 extra, to be able to detect errors. In theory you can also fix the errors if there is only one faulty bit, but most cases it is only detected.
This has its use on servers, once you get ECC errors you know your ram is bad and you can replace it rather than ruining your database or file storage with broken ram and realize the errors weeks later when they are already on your backups and disks.
Disadvantage is its slower and more expensive. Not all motherboards can work together with ECC RAM, since it requires some code in the bios to take care of the error checking and correction.
Weather you can use ECC RAM without the ECC in normal mode is byond my knoledge, but no reason to do so since its slower and more expensive. Well, i could think of one application, getting hold of an old company server that you want to use the RAM from. Anyone know if its possible?
Registred Memory
Registers delay memory information for one clock cycle to ensure all communication from the chipset is collected by the clock edge, providing a controlled delay on heavily loaded memories
stolen from
http://www.simmtester.com/page/memory/show_glossary.aspSo in short registered memory is slower but more relyable, again cant be used in any motherboard.
