There are generally two types of capacitors: polarized and bipolar. Polarized capacitors have a positive and a negative terminal; bipolar capacitors don't. In polarized capacitors the insulator between the plates is usually an electrolyte; hence the name electrolytic capacitors, or electro's. The electrolyte enables manufacturers to create large value capacitors with small dimensions. That's why you'll always see electro's with relatively large values: 1uF and above.