1: A block is what you are currently reading.
If I write a paragraph, or post a photo or link to a website, that's a block. A block technically stands alone: It rests in the channel you choose, but (if it's public) it can be connected to any other channel, by you or someone else.
This block is public as long as I create it within a public or closed channel.
2: The differences between the different types of channels are,
You can tell the difference by the color of the channel title. This channel is titled "are.na tutorial" and it is in blue.
See the comments on this block to understand the differences between public, closed and private channels.