Miners are system resources that operate the blockchain framework. These systems perform algorithms to verify transactions and include them in the parent blockchain to essentially act as a distributed network that keeps the cryptocurrency alive. They're shared computing resources that pay proceeds to those offering their CPU, bandwidth, etc.
Miners come in many shapes and sizes.
You can invest in Cloud Miners, Hardware Miners, Desktop Miners (for offline and online operation), and Browser based miners.
I have yet to venture too deep into the cloud, hardware, and desktop miners, but have been leveraging browser miners where available.
There are costs associated with miners, but those costs are primarily associated with the electricity and bandwidth required to run the software. You may incur additional fees for subscription based services, such as Cloud Miners.
Other costs would include the price of the hardware needed to run.
Desktop miners may seem like the way to go, but if you run full-bore, then you need to also account for cooling systems. This increases both the hardware and electricity costs.
Of course, there are always people trying to make money off others. I would suggest you do your own research, and if it is free, then buyer beware... Some sites offer a free starter package and are legit, but do not allow you to withdraw until you have made at least 1 minimum purchase. Other sites have had issues with providing payouts, but since some of those are free, I figured "what the hell" and signed up anyway. I can tell you this, right now, I'm still learning, so investing in any paid option isn't anywhere near my radar.
There are sites out there that rate the risk and identify scam vs legit.
caveat emptor: I have not fully vetted all miners that I currently use, since I'm not investing in them. I will at least identify those I have.
Below is a list of browser-based miners:
Verified:
Unverified: