What's the difference between a parish and a mission?
Fundamentally, there is no differences in the congregations of either. Both have communities that gather together for corporate worship and support the life of their church. The differences come more at the operational level. A parish is a church that can financially support its own operation. They also have a greater degree of independence in how they operate. For example, with the guidance of the Canon for Congregational Development, a parish selects and hires its own rector. The rector is the title for the priest-in-charge of a parish. The elected lay officials are called a Vestry and vote on financial matters concerning the church as well as function as a council of advice for the rector. Parishes also contribute a portion of monies they receive each month to the diocese as a part of mutual ministry support. That money is used by the diocese to, among other things, support congregations that cannot financially support themselves.
A congregation that cannot sustain itself financially is called a mission. Missions receive financial support from the diocese to operate. Because the diocese is supporting them, the Bishop also exercises more control in selecting the priest-in-charge of the congregation. A priest leading a mission is called a vicar. Another difference is that the group of elected lay leaders is called a Bishop's Council. When a mission can financially support itself for a period of time, they can petition the diocese for parish status. It's also possible that a parish can decline in membership, be unable to sustain itself, and may revert back to a mission status.
In our diocese, not all missions have permanent vicars. Some missions may have supply priests who celebrate the Eucharist with them on Sundays but are not with them throughout the week. Something new in recent years is the addition of bi-vocational priests. These are people who have not gone to seminary, but have been selected by the Bishop to be educated through the Iona program. This is the three-year formation program that is ran by the diocese for the formation of deacons and bi-vocational priests. Once ordained, these priests are assigned to non-stipend positions as priests-in-charge of missions that cannot support a full-time vicar.