They have entered our language. Everyone uses them. The terms, particularly with "byte", are almost commonplace. By convention multiples greater than one should be specified with a capital letter and those less that one with a lower case. Thus "Km" is correct but "km" is not.

The terms and their mathematical values are: (for those that "tend to infinity")

And going in the other direction, we have the terminology and amounts for numbers that "tend to zero" less than 1 (unity) but never truly reaching 0 - although the amounts can be so tiny that they can mostly be considered zero for all but scientific reasons. Note the terms less than 1 are written with a lower case letter. So you might have 1Mb but it takes 20mSec to fill it. Because there is repetition in some of the letters, micro is represented by the Greek letter "μ" (mu, pronounced 'myoo') similar-looking and often written as English "u".

Some find it useful to view a direct comparison of the above - it provides perspective on the scales...

 Approaching Infinity <--------------|--------------> Approaching Zero

										0.000,000,000,000,000,000,000,000,000,001  quecto  10^-30
										0.000,000,000,000,000,000,000,000,001      ronto   10^-27										
										0.000,000,000,000,000,000,000,001          yocto   10^-24
										0.000,000,000,000,000,000,001              zepto   10^-21
										0.000,000,000,000,000,001                  atto    10^-18
										0.000,000,000,000,001                      femto   10^-15
										0.000,000,000,001                          pico    10^-12
										0.000,000,001                              nano    10^-9
										0.000,001                                  micro   10^-6
										0.001                                      milli   10^-3
										0.01                                       centi   10^-2
										0.1                                        deci    10^-1
										1                                          Unity    10^0
										10                                          Deca    10^1
										100                                          Hecta   10^2
										1,000                                          Kilo    10^3
										1,000,000                                          Mega    10^6
									1,000,000,000                                          Giga    10^9
								1,000,000,000,000                                          Tera    10^12
							1,000,000,000,000,000                                          Peta    10^15
						1,000,000,000,000,000,000                                          Exa     10^18
					1,000,000,000,000,000,000,000                                          Zetta   10^21
			1,000,000,000,000,000,000,000,000                                          Yotta   10^24
		1,000,000,000,000,000,000,000,000,000                                          Ronna   10^27
1,000,000,000,000,000,000,000,000,000,000                                          Quetta  10^30

Many of the above grouping terms are bandied about with little regard to the sheer size of the number. Consider the following:

There are 86,400 seconds in one day. Assuming an average life span of 80 years, each of us has 2,522,880,000 seconds to live. Sounds quite a lot, but consider this: If you are middle-aged, you have around a billion seconds left to live. Can you hear ticking?

But what is that Billion in comparison?

• A billion seconds ago Britain was wracked with the poll-tax riots and unemployment was at an all time high in the "loads-a-money" era.
• A billion minutes ago Jesus was alive.
• A billion hours ago our ancestors had only just learned to use rocks to manipulate their environment and their global population numbered, probably, just a few thousand.
• A billion days ago no mammal walked on two feet.
• A billion years ago multi-cellular, complex animals had recently formed from single-cell creatures. Around half as much time again would pass before recognisable animals evolved.
• A billion decades ago there was no Sun or Earth. This region of our galaxy was populated by super-giant stars that lived short lives (~500 million years) and died incredibly violent deaths to seed a vast dust cloud that would eventually become the Solar system including all the planets and You.
• Two billion decades ago, *currently accepted theory of the Universe's origin says there was nothing. The entire universe was probably an (almost) infinitely hot, infinitely dense, infinitely small dot and even the numbers mentioned here can only approximate those required to model it with today's super computers.
• But... Two billion Pounds ago was only 26 hours and 24 minutes, at the rate our government is spending it.

hmmm. >:o/

* The big bang theory is in trouble. Recent revelations have produced fairly concrete evidence of no universe expansion. This article is fairly concise.