What is the difference between a solemn vow and a simple vow?

A solemn vow is one that the Church recognizes as solemn, otherwise it is simple. Temporary vows are always simple. Solemn vows are always permanent. However permanent vows can be either solemn or simple depending upon the religious community. All monastic and mendicant orders take solemn vows. These are strictly cloistered communities and most semi-cloistered communities. Active communities of religious orders take simple vows. The difference between the vows is in how much it bind the bearer. Those under solemn vows can only be released by the pope, while those under simple vows can be released from them by the bishop.