The power coins is what allows the power rangers to morph. There were originally 6, Zordon had five and Rita managed to get one.
When the Dragonzord coin was destroyed Zordon and Alpha created the Tigerzord coin. 
These coins were almost always kept in their power morphers. The two main exceptions are when they give all their power to one ranger, like they did with Billy while fighting Madam Woe, and when they use their power crystals.
The power crystals were summonded after Zordon thought it was impossible that Jason would be able to destroy giant Goldar and King Sphinx by himself. The power crystals could find any ranger if he was in danger. They were used as the joysticks to the zords and were created by taking their power coin and bringing their other had away from it.
