What is a Church?

I'm having trouble with the notion of a church. The church seems far less problematic - those who are 'in Christ', the body of Christ, etc. But what is a church? There seem to be lots of possibilities:

1. Those people, some of whom may be 'in Christ', who attend public worship at a particular building? Often called the congregation.

2. Those people who attend public worship together (there may be others who attend different services in that building)? Often called a congregation.

3. A group of Christians focused around a priest/pastor?

4. A group of people meeting together to participate in God's work of justice and mission (New Way of Being Church or Cell Church)?

There are others. I guess that numbers 2 and 4 seem most important. We should be treating them as the atoms of the church and wondering how we can support them in their development and interaction with other atoms. This means that one church building and one priest could support several different 'churches' within it. They may interact and overlap at times but at other times they will develop their own particular ethos and style (within the parameters of the catholic church) and will provide a home for those whose path to God lies through their particular way of approaching him.