Can someone explain in very simple words (non technical) what EVM is? Is it something tangible, like a computer, that all nodes in the Ethereum network have access to? If so, then where is it located? If not, then what is it? A software that all nodes use? How would you explain it to your grandmother? Thank you!
EVM can be thought as a single powerful computer. The computer can be accessed from the outside world only through nodes.
The biggest different between a 'regular' computer and EVM is that EVM is not physically located in any single place - it is many not-too-powerful sub-computers working together to form a big-ass mega computer.
However, this big-ass mega computer is not mega because of its computing power or speed. It is mega because of its reliability and distributed governance (decentralization): no single entity has the power to take it offline or change how it works. All of its results are 100% predictable (deterministic).
Imagine you have a million-dollar bike and don't want anyone to steal it. When someone adds a lock to the bike the bike is more difficult to steal. However, the lock's owner can just unlock the lock, take it with him and walk away - this isn't very safe. So, we get someone else to add also his lock. It's already much harder for any single individual to steal the bike if there are locks from two different owners. The more locks you add the safer it gets.
Bike locks are analogous to EVM's nodes. More nodes does not bring more processing power or more speed to the computer. But the more nodes there are, the safer the system is - no single entity can then alter it.
Quoting the yellow paper:
The execution model specifies how the system state is altered given a series of bytecode instructions and a small tuple of environmental data. This is specified through a formal model of a virtual state machine, known as the Ethereum Virtual Machine (EVM). It is a quasi- Turing- complete machine; [...]
In other words, the EVM is an abstract model of how a "system state" looks like and how this state is updated. All (full) nodes in the Ethereum network follow the rules defined by this model, calculate the system state on their machine and (as long as they use the same input data, i.e. the same transactions), arrive at the same result.