Electrolytic capacitors are polarized with both a positive and negative. On through-hole capacitors the negative leg of the capacitor is identified with the negative symbol printed on the capacitor body and the positive leg is often longer. This design makes identification easier when assembling.
If an electrolytic capacitor is installed with the incorrect polarity or on an AC circuit the capacitor often bursts. On the top of electrolytic capacitors there is often a cross. This is a designed weak point to allow the capacitor to burst safely.
Capacitance is measured in farads. The value of capacitors is usually expressed in pico-farads and nano-farads with a maximum voltage marked on each capacitor.