It’s part of the hierarchy of the church.

Diocesan priests “work” for the bishop in a particular geographical area and are in the “corporate” hierarchy of the church.

Religious orders are sort of independent from the the church hierarchy and report through to the leader of their order, at a global level. They often focus on specific things and may have different vows. Franciscans are known for their work with the poor and personal vows of poverty, for example. Also the order is a community that has its own governance.

I have friends who are in a similar organization as nuns. They govern themselves democratically and globally. It’s pretty amazing - we helped them setup their real-time voting system to manage their community. Each group is different.

You can (sort of) divide Catholic clergy into diocesan priests, who spend their careers managing the clerical hierarchy of a specific region, and religious-order priests, who belong to religious orders within the church --- the Jesuits, Franciscans, Augustinians, Dominicans, etc. The "religious" Clergy are thought to be in some sense less tied up in church politics.

There are a few different orders within the catholic church with some of their own intellectual, practical and traditional differences. Most popes don't come from any of the orders. The last two popes did. That's historically odd. Francis had been the first one from his order ever, even though it's the largest one.

To the other useful answers I just want to add that if you think about monks, nuns and friars, that covers a large portion of what a Catholic religious order looks like.