Socket 7 is a 321-pin PGA socket originally designed for Intel's line of "Pentium MMX" processors, circa 1996 or so. The physical characteristics are identical to the older Socket 5; only the electrical specifications changed. The main changes being the split voltage options and a disipation of 17 watts, versus 15 for Socket 5.
Socket 7 originally operated at 60 to 66 MHz (with some rare boards dipping down to 50 MHz) with multipliers usually reaching 3x. Most Socket 7 boards offered the split voltage option, with Vio at 3.3v and Vcore at between 2.7v and 3.0v.
There is another incarnation of Socket 7, which correctly called Super 7, that supports faster bus speeds (up to 112 MHz in some cases) and more varied split voltages (some going down as low as 2.1v Vcore).