The missional church grows out of a new reforming movement in Christianity that began in both England and the U.S. in the 1980s.
At that time in particular church leaders realized that "Christendom" (an environment of a predominantly churched culture that set the cultural markers) was over. Most church identity and life had been molded in the world of Christendom where the church was seen as primary, and because of its primary influence in the culture little effort was needed to attract people who often inherited their faith and church loyalty. (This could be the case whether or not one was any particular kind of church or whether or not one was Christian; the cultural influence was the same). Any mission to others, especially in other parts of the world without churches, was seen simply and secondarily as a program of the church.
Thus were born individual missionaries of the church.
But in a post-Christendom world, as in a pre-Christendom world before the Roman Empire coopted the church, Church is not primary; instead now, as in the first 300 years, Mission is primary. However, it is not the church's mission; it is, rather, the Mission's church. This is the difference between missional church and a church that does outreach programs. The missional church is an effect, a creation itself, called into being out of a deeper identity and sense of mission. That mission comes from the very "missio" nature of God, using the Greek word for "being sent." God sends God's self into the world, which is also known as incarnation. And especially is God's self sent into the world of suffering, of the poor, the outcast, as evidenced by the presence of God in and through Jesus. As God is, then, so should be the church.
Rather than creating and sending out missionaries as before, the church at this point thus becomes itself wholly missionary.
But the forms and practices of most church life today in the European and North American context, because they have been created by and in the context of Christendom, often stand in the way of the church as a people being sent into the world together to gather with the suffering, the poor, the outcast. The missional church, which also comes in different forms itself, including more organic than organizational relationships of people, turns the Christendom model inside out and upside down. Now the church only exists when it is gathered from out of the midst of the very vulnerable ones where God is already at work. This is its mission: serving others, finding God at work there, and joining with God and all, and in gratitude celebrating through worship that refreshes the spirit for the cycle of more serving, more finding, more worship. See how that inverts the "usual Christendom model" of resources and identity and church life that focuses first on worship celebration, then the "finding" one another and belonging, being taught, only then serving others.
But if that is the case, that the church only exists when it is gathered in the midst of being with the most vulnerable, then all its forms and practices, its leadership, its resources of people and space, its activities, should be directed toward and reflected by that mission.
Finally then, as opposed to the centuries of Christendom culture and history, it is the mission that converts the church.