Stated as what you share, because that is what every other difference follows from.
Your site sits on a machine with many others and the CPU and memory are pooled. It is by far the cheapest, and it is genuinely enough for a brochure site, a small WordPress site or a new store. The trade is that a busy neighbour can affect you.
A slice of a machine with CPU, memory and disk reserved for you alone. A neighbour cannot take your resources. You choose the size up front and changing it is a deliberate upgrade, usually with a short reboot.
Reserved resources like a VPS, but the capacity is drawn from a pool rather than one physical machine, so it can be resized without rebuilding and survives the loss of a host node. You pay for that flexibility.
A separate question from all three. Managed means we patch the operating system, monitor it and fix it at three in the morning. Unmanaged means root access and it is yours. Do not buy unmanaged unless somebody on your side is on call.
Signs you have outgrown what you are on, in the order they usually appear.
Fast most of the day and slow at a particular hour is the classic shared-hosting symptom - either a neighbour or your own traffic peak. Reserved resources fix it; a faster theme does not.
Shared plans cap CPU, memory and processes. If your control panel shows you hitting those limits, you have outgrown the plan rather than misconfigured it.
A specific PHP extension, a queue worker, a non-standard database, a background daemon. Shared hosting cannot give you those; a VPS or cloud server can.
Once an hour offline is a real loss, the arithmetic changes. That is the point at which cloud hosting, which survives a host node failing, stops being an indulgence.
Roughly, and in that order - the gap between them is smaller than most people expect.
Low enough that cost is rarely the reason to leave it. Current prices are on the web hosting page.
Usually several times shared rather than ten times. If your site earns anything, the gap is small against an hour of downtime.
The difference is somebody else being responsible at three in the morning. Price the alternative - your own time, at the worst possible moment - before deciding it is expensive.
Plans and prices for each of the three.