http://www.tomshardware.com/answers/id-3246929/seasonic-s12ii-520w-good-system.html
There are 3 categories.
N0on-modular - This are the least expensive category and it means that all PSU cables are "hard wired". Advantages are cost, increased efficiency, least possible failure points, lowest resistance... downside is all the cables you don't use are laying inside ya Case looking ugly and may impede air flow
Modular - This are the most expensive category and it means that all PSU no cables are "hard wired". Disadvantages are increased cost, lower efficiency, most possible failure points, highest resistance... upside is you don't have any unused cables inside the case cluttering things up.
Hybrid Modular - This is the proverbial...