EVM is the Ethereum Virtual Machine, the protected, distributed "sandbox area" (virtual machine) where contract execution occurs, replicated on every node in the network. EVM is also the name of the bytecode that runs on the virtual machine -- what Solidity, Serpent and LL get compiled down to before deployment.
The EVM is a virtual machine that can execute the instructions that comprise Ethereum smart contracts. It is specially optimized for blockchain usage: all operations are completely deterministic (which is essential for nodes to reach consensus on the resulting state changes of the execution), memory and computation are carefully tracked, and certain cryptographic operations are "built in".
EVM bytecode is the instruction set of the Ethereum Virtual Machine, which is produced when compiling the higher-level languages, Solidity, Serpent and LLL. The term EVM is also used to refer to the bytecode itself.
