I think you're either asking about a monospaced typeface – where each character has the same width – or the tabular lining opentype feature, which makes numbers the same width in typefaces which havewhere this feature available, which. This is useful for displaying alot of numbers in tables for instance, or when numbers need to align perfectly across multiple lines.
Some good examples of quality monospaced typeface families are the Decima Mono Pro or the PF DIN Mono.