Define bus and memory transfer?

A bus is a set of common wires that carries information between the units of a computer. A bus transfer moves the content of one register to another register over this common path, and it is written as $R2 \leftarrow R1$.

A memory transfer is the movement of data between the CPU and the memory. A read brings a word from memory into a register, written $DR \leftarrow M[AR]$. A write stores a register's content into memory, written $M[AR] \leftarrow DR$. Here AR holds the address and DR holds the data.