One other thing, in addition to what's been said above: When you look at the description of the channel, look for wording like "additional fees may apply." That means there's probably a subscription. While you may not be charged to add the channel to your Roku, you may have to pay to use it, or keep using it (Netflix, Hulu Plus, Amazon, etc). That's what the "additional fees may apply" thing means.
yes, and there are a few channels with some free and some paid content also, so "additional fees may apply" could refer to a subscription, or it could refer to getting free ad supported content vs paid ad-free content, or like the history channel, a few free episodes until you sign in with your (if you have one) cable provider's info.