Ethereum is the fuel (the gas) for the Ethereum Virtual Machine (EVM). The EVM is a virtual central processing unit that executes Turing complete code to compute anything. It is deterministic, so it follows unambiguous rules.
There are two types of nodes on the Ethereum network: full nodes and light nodes. A light node does not hold any historical data or execute the smart contracts, whereas a full node does both of these. Metamask Wallet is an example of a light wallet because it holds no information about the blockchain outside your private key.
The difference between full nodes and light nodes is that the former maintains the entire blockchain’s data, while the latter only focuses on certain aspects. As a result, light nodes are faster than full ones because they have less information to compute with. However, being faster means being more vulnerable to certain types of attacks since there is less historical data to work with.
Light node wallets provide an easy way for users without enough computing power to access the network, but their lack of functionality limits them. This trade-off allows anyone with basic technical knowledge to use Ethereum without running a full node.