A tiara is semi-circular and covers just half the head. Whereas a crown is circular and covers the head completely.
A tiara is usually worn by females whereas a crown can be worn both by males or females.
A tiara isn’t studded with precious gemstone whereas a crown is elaborate with precious and rare gemstones.
One doesn’t have to be a royal to wear a tiara. Beauty pageant winners and brides usually wear them across the globe. But a crown is a representation of authority and is, thus, generally worn by monarchs only.