how does Roku work? ?

Roku, as some have explained here, is a box for streaming channels. It is the best on the market because you can subscribe to many different channels and also get free channels. 

You do not have to pay for every channel. I watch some free ones and I pay for things like Netflix on it. 

I suggest watching videos about Roku on Youtube to get an idea about it. 

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Roku turns your TV into a "smart" TV.  It has many apps available (way more than most smart TVs).  Each app offers different content which may or may not require a subscription.

For example, Tubi, Crackle, Stirr, Plex, Roku Channel offer free TV shows and movies.  Note that many are older shows such as "Petticoat Junction", but some offer more current content.  Many seem to offer the same "channels".

Then, different networks offer their own channels, ex. CW with on demand content.  Some also offer live content.

Some apps require either a subscription or will let you watch them if you have a current cable/satellite account.  For example, I have Spectrum for our main TVs, but I can watch all my channels live on my Roku.  You can do Sling w/o the Sling box.

There are a lot specialty apps.  I like WPT Poker channel.  We have the Cat music channel.  Some are mixture of free or paid content.

You can also get subscription apps:  Netflix, HBOMax, Disney +, Amazon, Hulu, etc.  You pay for those separately.  Since we have existing accounts, you just need to set them up in Roku.

The main issue I have is their remote.  Its super simple (and I like simple), but there's no page scroll.  If I'm on the Spectrum app I have to hold down the down arrow to move through the channel listing.  There's no volume control.  

You can search for movies and TV shows and it will show you if they are available and on what app, even if you haven't add it.  

A Roku is a device, not a service.  I would describe it like a tablet that connects to your TV.  You download apps and then you open the apps and watch what is offered in the App.    There is a Pluto tv App, Tubi App, and Roku tv App you can download for free and watch 100's of channels you've never heard of for free.  You can get all your familiar channels like HGTV and Fox, but they are part of subscription services like Sling (App) or the Youtube TV (App).  

You can download the Pluto App and Tubi App on your phone\tablet to see what you can get for free.  *Note: Roku calls Apps "Channels", but it isn't an accurate description and confuses people.  

Roku is a streaming device. By itself, it won't let you watch TV channels. You have to subscribe to streaming service(s) to watch TV shows. I don't know much about these services so I don't know how much they channels they do or don't offer. You'll have to do some research on the major services to see what works best for what you want to watch. cordcuttersnews.com seems to be a good website for learning about the difference services and their pros and cons.

There are generally free streaming services out there. However, they are ad-supported just like regular paid TV service.

A Roku is just a piece of hardware that allows you to stream content to your TV.  You'd get the same functionality built in to a smart TV, Xbox, Fire stick or other similar device.  There are some free streaming providers, like Crackle.  You can get them through a Roku, a Fire Stick, a smart TV or a computer.  HGTV is not free.  Fox has free programming that can again be accessed through an app or web interface.  The Roku is just something that allows you to install apps and stream video to your TV.  You can do the same thing with a laptop, phone or tablet if you choose, although the Roku (or Fire Stick or Apple TV) has apps for it that make this much more streamlined and simpler than configuring your phone, tablet or computer.