First, as noted earlier, the church is made up of the saved (Acts 2:47). That puts the church in a unique relationship to the rest of the world, the un-saved. The church is identified as "the pillar and ground of the truth" (1 Tim. 3:15). As such the church not only stands on the truth but it also upholds the truth. God's plan is for His will, His truth, to be communicated throughout the world by the church (Eph. 3:8-11).
The apostle Paul wrote, "For since, in the wisdom of God, the world through wisdom of God, the world through wisdom did not know God, it pleased God through the foolishness of the message preached to save those who believe" (1 Cor. 1:21). It is the church's mission to preach/teach God's message of salvation to "all the nations" (Matt. 28:19-20), to "every creature" in "all the world" (Mk. 16:15-16), "to all nations" (Lk. 24:47), "to the end of the earth" (Acts 1:8). This is about as basic and fundamental as it gets in understanding what the church is all about. The church is to teach God's word relentlessly to all people everywhere.
The world outside Christ is lost in the condemnation of sin. They need the Savior. The church is responsible for spreading the good news - the "gospel" (Rom. 1:16) - about salvation in Christ.
The church is to glorify God in all that it does (1 Cor. 10:31), part of which is to express its adoration of God through worship that conforms to His will (Jn 4:23-24). Another way the church should glorify God is by serving Him through the good works that are keeping with His will (Eph. 2:10; Tit. 2:14). The church should be a shining example of godliness before the world around it (Matt. 5:13-16) and demonstrate the love of Christ before it (Jn. 13:34-35).
What's the church all about? It's about serving God, glorifying Him, teaching His truth to all mankind. The church is here to teach the lost and nurture the saved, to help them grow spiritually and prepare them for eternity in Heaven. Truly, there is no more important entity in the world than the Lord's church!