Hydrogenated oils were first available in the United States in 1910 (I think it was first introduced in France but I'm not sure) and by 1911 Crisco was on the market. Margarine, which was originally made using lard and suet, began to contain more and more hydrogenated oils. Depending on the type of hydrogenation used, partial or complete, different products can be made. Complete hydrogenation yields a solid fat at room temperature while partial hydrogenation results in a softer fat which could be used for spreadable margarine for example.
Hydrogenation has many benefits (as far as food companies are concerned anyway): It gives the oils an increased melting point, making them more solid as well as increasing the oil's resistance to oxidation giving it a longer shelf life. Basically, during hydrogenation, fats are forced to bond with more hydrogen atoms than they would normally. The fats become saturated with hydrogen. Essential fatty acids and polyunsaturated fats are transformed into saturated fats. Any health benefits provided by the polyunsaturated fats are lost. The process also creates trans-fats which are very detrimental to one's health.
In order to acheive hydrogenation an oil is placed under high temperature and pressure and a metal catalyst is added. Hydrogen gas is then injected into the mix. The whole process takes about 8 hours.