Power Supply Unit

There are three type of SMPS called at, atx, and btx

AT SMPS

Advanced Technology SMPS has a 12-pin power connector they were used in Pentium-I, Pentium-II, and Pentium III CPUs.

ATX SMPS

Advanced Technology eXtended has a 20-pin power connector called ATX power connector. They were used in Pentium-III, Pentium IV and AMD CPUs.

BTX SMPS

Balanced Technology eXtended SMPS have a 24-pin power connector and 15-pin SATA power connector and is used in Dual core, Core2Duo, Quad Core, i3, i5, i7 and the latest AMD CPUs.

There are three type of Gaming SMPS called Non Modular, Semi Modular, and Full Modular.

Non Modular SMPS

The SMPS come with all possible cables attached to it 

This has the advantage of ensuring you're not going to need a cable. The downside is that it's VERY messy for cable-management. Trying to find a place for all the excess cables can be a nightmare. Bad cable-management can mean the difference between good or bad airflow and consequently the operating temperature of your system. 

Semi Modular SMPS 

Only the essential cables come pre-attached. Usually this means, 24Pin ATX, a single PCIe, 8Pin CPU, and maybe one or two peripheral power cables (SATA/Molex). There's less clutter and it

is definitely much better than non-modular and a cheaper solution than going full modular.

Full Modular SMPS 

Comes with no cables pre-attached. You only attach the ones you need.

It is more expensive, but also gives you more options, allowing you to only install the cables you need and keep airflow and cable management to a maximum. All newer generation HALE SMPS are fully modular.