The type of rice depends on the degree of processing dealing with its multiple layers: brown rice includes the bran, germ, and endosperm - the bran and germ are nutrient rich, but have tougher texture and a characteristic earthy color; white rice is generally just the endosperm - the chaff, bran, and germ removed - pretty much just the starch part of the whole grain.
Rice being mostly starch, is composed of semi crystalline granules. If exposed to enough heat and moisture, these starch granules swell and soften, loosening that hard, crystalline structure, a process known as gelatinization. When rice is fully gelatinized, it is soft and palatable; otherwise it feels crunchy (undercooked).
After gelatinization, continued heating or agitation begins to break down those swollen starch granules, leaching amylose and increasing the viscosity of the surrounding liquid. This process known as pasting results in sticky and mushy rice (overcooked).
Rice contains two main types of starch: amylose and amylopectin, both large polycarbohydrates that store glucose. Amylose is less soluble in water than Amylopectin, therefore the latter contributes more to the pasting effect. Rice grain varieties that are relatively high in amylose tend to be more separate when cooked, with less sticking between grains, while rice with a higher relative amylopectin content tends to produce stickier rice with less separation between grains.