First, check the link above and read. Then I would just add in this to explain a bit further. I only read into all this stuff because my son is a nuke in boot camp right now.
There are 3 rates you might be placed into by the Navy. MM-machinist mate, EM-electricians mate, ET-electronics technician. Needs of the Navy is the decision maker, along with your ASVAB scores, but they tease you with the thought that your preference will also influence the placement. haha.
Anyway, if you sign up for MM in your DEP contract, you are for sure going to be MM, it won't change. If not, you may be rated any one of the three.
If you are MM, and you do really well in nuke school (A school, power school, and prototype) you might be selected for ELT school or may get to choose that on your own (??don't know)... but it is an additional 13 weeks of training to be completed after graduation from power school, the final portion of your nuke schooling.
MM with ELT is from what I have read, a pretty neat option, and can give you a slight edge from what I can tell.
I don't know much more than that, so don't ask me haha... but that is what I have learned so far.