Computer processors are often defined in terms of the ‘word’ size they can handle. In computing, the term ‘word’ refers to the block of data (specified in number of bits) that can be manipulated in a single clock cycle.
So a 16-bit processor has a word size of 16 bits, a 32-bit processor has a word size of 32-bits and a 64-bit processor has a word size of 64-bits. From this, it’s clear that a 64-bit processor is able to handle more data in the same clock cycle and is therefore more efficient.
Newer processors are backwardly compatible. 64-bit processors, for example, are able to detect 16-bit and 32-bit applications and process them appropriately.