what is the church?

Recently was in the Seattle area and while hanging out with my friends, we went to their home church. It was a med-sized place, 4 sets of greeters before the sanctuary, open spaces and pretty modern. One of the greeters though said something that caught my attention for some odd reason, "Welcome to Church." Now they said that, but it was something that stuck with me as I entered. Stuck through worship. During offering they mentioned stuff about church related and it still stuck out. Then the sermon started. Sermon was extremely good. I thoroughly enjoyed it and there were some really awesome points, one of the best ones was that, we need to "scourge our schedules." As I was listening though, there was philosophical question that begged to be asked and answered. All the while during the sermon, he said that people will not be tired as you dedicate yourself to the church, that the church is a place of restoration, that the church was one of the most important pieces of our lives and if we just committed ourselves to regularly going to church our lives would be so much better. How is church being defined? In this example it almost seems as if church was apart of the trinity. I know that is not the heart motive of the pastor however, a clearly defined definition from the start would have helped. Are we talking about the actual service, physical building known as a church, or the body of Christ that is preparing itself for the Lord's return? For the comment section: how do you define "church" and how important do you think an actual building and service is?