Java bytecode is normally contained within a .class file and can be executed by Java Virtual Machine (see Java). It is an intermediate stage between a human-readable code and machine code, a compiler is used to turn normal code into bytecode.
Bytecode consists of the predefined set of instructions which are known to the Virtual Machine. Within the .class file these instructions are stored in binary representation.
The .class files are rarely distributed by themselves, but are combined in .jar or .war files.