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In brief, in the Catholic Church there are three levels of Holy Orders, which means ordained clergy: deacons, priests, bishops. So deacons (even ones who have wives and children, and ones with non-church jobs to support their families) are clergy, not laity.

Although many Catholics see deacons as assistants to the priests, they are directly answerable to the Bishop, not just to a priest they may happen to be assisting. In other words, deacons are appointed by the Bishop, not by a priest or parish. And alhough most deacons work at parishes, some don't.

Deacons have liturgical roles that cannot be filled by laity. They can preside at a Communion Service with pre-consecrated elements but they cannot celebrate Mass). At Mass, they can proclaim the Gospel and deliver the homily. Other roles are summarized (in fun) by "hatch, match and dispatch". They can baptize, officiate at weddings (but not say a nuptial Mass), and preside at funerals and interments (not not say a funeral Mass.)

Married men can become deacons, bnut deacons cannot marry. (The order or sequence is what's indicated here: you can be married and then become a deacon, but you cannot first become a deacon, and then marry.) Married deacons, if they become widowers, may not remarry.

There are other small differences between deacons and priests, but those are the main ones.