KB or Kilobyte = 1024 Bytes, or roughly a thousand
MB or Megabyte = 1024 x1024, or 1048576 Bytes, or roughly a million
Note these numbers are for uncompressed data, just to give you an idea on raw data throughput. With compressible data, throughput can go up by a factor of 2 or 3 over these ratings. However, because graphic images on web pages are already compressed, the real multiplier for web browsing generally works out to around 1.5 to 2x the above listed rates. So divide the times by 2 to approximate real world download times assumming 2 to 1 compression. Faster is better! It is easy to cost justify that $100 for a 56K modem if you value your time. Amazing but true, a large number of web surfers out there still use those tired 14.4 modems.
Life is short. Get a V.90 modem fast!
Two rows of data are shown for V.90 modems because your connect speed will vary depending on the quality of your phone line and distance to your "CO" or Central Office (local telephone facility). A pretty good line should get about a 48 to 50K connection. The Telephone Network was designed to carry voice traffic, not the flood of data transfer it is being used for today. It is generally claimed that people who live within 3 1/2 miles from their central office will get significantly better connect speeds using a V.90 modem over using a V.34 modem.
DTE vs DCE speed. Modem shoppers in stores like CompUSA often ask why they should buy a 56K modem, when their modem already says "connected at 57600" or "connected at 115200". Well, this is a common misunderstanding of what the reported connect speed means. These speeds are the DTE (Data Terminal Equipment) speed, which is the speed of the connection between your PC and your modem. The DCE (Data Communications Equipment) speed is the bottleneck. The DCE speed is the speed between your modem and the other modem you connect to. Prior to 56K modems, the best speed you could get between two modems was 33600 bits per second. Now using 56K technology, rates approaching 50000 bits per second are achievable.
Asymetric Data. Notable is that the V.90 standard calls for 'asymetric' data rates, just as both K56flex and X2 did before the standard. This means that the send and receive data speeds are different when you connect in V.90 mode. Fortunately, high speed (up to 54Kbps) is realized in the downstream direction, which is good since this is where the bulk of the data is, downloading graphics, etc. from the internet to your modem. The upstream direction is limited to 33.6Kbps, which is OK, since most of the data web surfers send is mouse click commands, which don't require much data to be transmitted.