From an older thread:
Non-modular:
The PSU comes with ALL possible cables attached to it, ready to go.
This has the advantage of ensuring you're not going to need a cable.
The downside is that it's VERY messy and if you're into cable-management, it's
a nightmare storing away all the excess cables you don't need. Bad cable-management
can mean the difference between a good or bad airflow and consequently the
operating temperature of your system.
Semi-modular:
Only some cables come pre-attached. There's less clutter and it
can be pretty alright. However it's more expensive than non-modular.
Modular:
No cables at all. You only attach the ones you need. This is ideal for
tweakers and those with deeper pockets, as it's both more expensive, but
also gives you more options.