1. a building designed for public forms of worship, esp Christian worship
2. an occasion of public worship
3. the clergy as distinguished from the laity (regular membership)
4. (usually capital) institutionalized forms of religion as a political or social force: conflict between Church and State
5. (usually capital) the collective body of all Christians
6. (often capital) a particular Christian denomination or group of Christian believers
7. (often capital) the Christian religion
There is value and difficulty in each of the definitions above, but I think it's critical to understand and remember that when one person says "the church" or "The Church" that person often means one of the above definitions, while the person hearing it often thinks of a different definition. That alone causes as much confusion and disagreement as anything else.