It's actually quite difficult to define. Unrelated business income is income that is not related to the core business of ministry. Suppose you rent space in the church building to a zumba trainer to have classes. That's probably unrelated business income. But suppose you rent to AA; that's arguably a ministry, so not taxable.
Or suppose the pastor has his own house and you rent the parsonage—the rental income is probably unrelated business income, unless you are renting to a pastor from another church that does not have its own parsonage.
Or you have a web site where you sell CDs and podcasts of sermons, no problem. You also sell t-shirts with religious slogans but which do not specifically reference your church; could be taxable.
You operate a homeless shelter and collect money from the city/county to offset costs, probably ok. You run a youth hostel for tourists visiting your city and use the money to fund other ministries, probably UBI.
And of course, just because you have unrelated business income that must be reported, you might not owe tax if you have offsetting expenses.
<a rel="nofollow" target="_blank" href="
https://www.irs.gov/charities-non-profits/unrelated-business-income-tax">https://www.irs.gov/chariti...>
<a rel="nofollow" target="_blank" href="
http://www.umc.org/gcfa/tax-packet">http://www.umc.org/gcfa/tax-packet</a>
If your church is a member of ECFA they are a good source, or you can buy a personal membership (I think) <a rel="nofollow" target="_blank" href="
http://www.ecfa.org">http://www.ecfa.org</a>
