The people who answered are on the right track - that is to say not incorrect, but I wanted to expand just a bit.
The capital C Church is used in a title like First Baptist Church of Chicago, or Grace Church of Atlanta, and also in names of recognized denominations, such as Church of God. But another very common practice is "big C Church" used in the context of the universal body of Christ - all over the world - the entire collective group of saints.
For example "The Church is the bride of Christ, and the rapture is a picture of the wedding."
For centuries the Church has connected the number 3 with the Trinity and the number 7 with perfection and completeness."
"One of the clearest signs that Christ gave of end times is apostasy and false teachers in the Church."
When referring to local congregations, the word church is usually not capitalized. Again- these are examples only- "The church in Corinth was predominantly Jewish after 50 AD."
"The first church in Plymouth Rock was founded by Puritans from Holland". "Our church has the Lord's supper once every month."
This is a widespread convention that Christians use and understand in English speaking cultures around the world - Ireland, England, USA, Canada, New Zealand and Australia, as well as by many English speaking people of other countries.
I was a leader in an international church in Poland, and was in contact with foreign pastors from Denmark, Poland, Ireland, Holland, England and New Zealand and this is a widely used practice.