Internet speeds are measured in Megabits/Kilobits (mb/kb). Download speeds are measured in Megabytes and Killbytes (mB/kB). This is why you have downloads at 690kB/s (Kilobytes per second) on a 5 megabit internet connection. E.g. I have 12mb download speed and get 1.6mB/s downloads
Hope this helps.
Just for reference:
1 megabit = 1000 kilobits = 1,000,000 bits
1 megabyte = 1000 megabytes = 1,000,000 bytes = 8,000,000 bits (assuming we are using 8 bits per byte)
Source: Computer Science Degree Student
EDIT: A couple of things i should have noted:
mb and Mb (and kb and Kb) all mean megabits/kilobits (same goes for kB and KB, and mB and MB)
I used 1MB = 1000 KB in my example, this is a bit of a grey area but I know I would like to say 1KB = 1024 MB (as, with binary, you use base 2 numbers, so 1024 should be used as opposed to 1000 as 1024 is a power of 2), but apparently some people don't agree and the value of a KB has been changed to 1000B, with 1024B = 1 Kibibyte (some nonsense like this is probably what the UK government decided, idiots lol, i could say more but i'll just leave this here http://en.wikipedia.org/wiki/Kibibyte)
Finally, know that, when you are sold internet you are buying internet speeds of UP TO 5mb/s. In britain at least, OfCom (regulator) have rules saying how far outside of the target your speeds can be on a regular basis but you could easily be getting around 3.3mb/s, just something i thought i should point out
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