Greases have been know and used since biblical times or before. One of the first greases was olive oil mixed with lime to make a calcium base grease. Greases can have many base oils and many thickeners. Grease is basically an oil that has been thickened and will thin under shear. Often used in hard to lubricate bearing where oiling is not possible and the oil will not cling, grease must be replaced in the bearing void as it is displaced. Hence grease fittings to force new grease into the voids and the need to repack wheel bearings. Many greases are EP rated with most using a sodium compound or Moly Di sulfide as the agent to keep metal to metal contact from occurring under very heavy load. Some of the thickeners are for very special uses and not often seen, but one should be careful when changing grease base/thickener system as some are not compatible and bad things happen to the bearing when mixed. 
Most grease found in automotive supply houses are a #2 hardness grade. But some machines in the blacksmith shop call for other then a #2 hardness. Some lube points on Little Giants call for a softer grade and using the wrong grade makes the machine fail to run correctly.
LGI number ASTM worked (60 strokes)
penetration at 25 °C
tenths of a millimetre Appearance Consistency food analog
000 445-475 fluid cooking oil
00 400-430 semi-fluid apple sauce
0 355-385 very soft brown mustard
1 310-340 soft tomato paste
2 265-295 "normal" grease peanut butter
3 220-250 firm vegetable shortening
4 175-205 very firm frozen yogurt
5 130-160 hard smooth pate
6 85-115 very hard cheddar cheese

Grade 0 to grade 3 are pretty common in machine tools. The color of grease IE green, blue, red etc is mostly an advertising gimmick.

Written by Jeff Reinhardt