My understanding of google analytics is that it is a ‘free’ tool which gives site operators bird’s eye information on site traffic like the old fashion visitor counters all the way down to very granular information like what buttons users click on.
I have no idea what google tag service does. Based on a prior conversations I’ve had, I believe it has something to do with SEO.
If you don’t know what I’m talking about, use the developer tools in Firefox to view the HTML of the page you are on. In the header you’ll see javascript blocks. You’ll see google analytics and tag service on just about every site, often meta or amazon, and some with completely unidentifiable names. I imagine the latter are the non big tech third parties we accept with cookies, but I’m not sure about that either.
Google analytics is loaded by JavaScript. There are also other things like Google analytics that are also loaded by JavaScript.
Updating a website can take time, and usually involves someone with at least a passing knowledge of development.
Google tag manager is a service that lets you embed one JavaScript thing in your page, and then it will handle loading the others. This lets marketing or analytics people add and manage such things without needing to make a full code deployment.
It also lets you make choices about when and how different tracking events for different services are triggered.
It’s intended usage is garbage tracking metrics and advertising. Some sites are built more by marketing than developers, and they’ll jam functional stuff in there which causes breakage if you block it. These sites are usually garbage though, so nothing of value was lost.