As a streamer, it's really hard to get any sort of feedback on whether you're doing a good job. Whether this is a hobby, a fledgling business, or your full time job, you're probably tempted to look at your viewership to see whether you're making good or bad decisions. Is that worth doing? Deceptive? What do the different numbers even mean?
In general, a "viewer" is someone presently (or recently) consuming the video stream. So long as the actual video is playing, that person should be counted. Some browsers will unload tabs to conserve memory for tabs which are visible, which would stop the video player; prevent this by keeping the volume high enough that the tab shows a "speaker" icon on it.
An engineer who worked for Twitch confirmed that you can leave the video player muted and you should still count as a viewer. If the video buffers and the tab is not visible, the player may not resume playing, however, so to ensure that you continue to count, switch back to the tab periodically.
To minimize the bandwidth a background tab consumes, you can lower its video quality, as long as the stream has transcoding active.
A "chatter" is someone connected to the text chat. Most people will be both viewing and chatting, but it's possible to be either one without the other. Many people will lurk in myriad channels' chats, and bots often connect to chat without viewing the stream; these do not count as "viewers" but usually do count as "chatters".
When you raid another stream, the number of raiders is closely related to the number of chatters, but only those who actually "ride the raid". Details on exactly how this works are scanty.
This is not an official term, but the viewer count has one special case: the achievement required for a partnership application involves an average of 75 viewers, "excluding Hosts, Raids, and Embeds". All other definitions of view count include those subcategories, but for this specific calculation, anyone who has raided into your stream will not count; nor will people watching your stream using some forms of multi-twitch (which count as "embeds"), or those seeing your stream hosted elsewhere.
Note that asking your viewers to remove parts of the URL, or to click on something, or anything like that, has no benefit other than placebo. Focus on making the best quality stream you can, and don't stress about the things you and your viewers can't change :)
Remember, too, that the viewers who do not count to the 75 will still improve your stream's discoverability. Everyone is helping there and they don't have to do any work at all! There are a few other ways that viewers can go over and above though - see Discoverability tips for some ideas!
Honestly, I don't know what this figure actually means. It goes up whenever people watch your stream but I have no idea what "one view" actually is.
This one's pretty simple. Someone follows your channel, it goes up. Someone unfollows, it goes down. Someone gets deleted who had followed your channel? You guessed it, it's going down. So don't stress about losing a follower here and there; people's accounts get cleaned out if they abandon them.
If you're trying to qualify for Twitch Affiliate status, you need to have an average of three viewers for a month, and fifty followers. If you're trying for partnership, you need seventy-five "right-here viewers" for a month.
As a measure of channel growth, all of these figures are very noisy. You can't look at today's stream and yesterday's and then extrapolate from there. Even across a few weeks, there is considerable noise; and there are busy and quiet times of year. Comparing your stats from the same time last year is likely to give reasonably useful results, if you can be that patient.
When you're deciding what to do. When you're deciding what time to stream. When you're deciding pretty much anything about your stream! Stream what works for you, don't try to "follow the numbers"; don't play the most popular games, or the most popular days of week, just for the sake of "getting viewers". Be what you can best be, even if it looks like you have zero viewers; stick with it for at least a few weeks or a month before you decide that it's not working.
Especially, don't feel disheartened because nobody's in chat today, or the viewer count is showing at zero. Stream anyway, and address the invisible lurkers, because you never know who's watching the stream logged-out, or is checking out the VOD afterwards!