(Not for nerd readers although you’re welcome to stick around and tell me what I got wrong in comments)
What’s broadband? Good Question. There is no consensus on the answer except that it involves a connection which is better than dialup. Most people use the term to mean a connection which can be setup for an indefinite period of time (persistent), which has enough bandwidth (discussed below), and low enough latency (also discussed below) and jitter (below) for whatever use they intend to make of the connection. In practice the requirements for minimal useful broadband keep ratcheting upward since Internet services are designed for users with about 50th percentile capabilities.
What’s bandwidth? That’s an easier question. Bandwidth (in its common but not engineering use) is a measure of how much data can be delivered over a connection in a given period of time. Usually bandwidth is quoted in bits per second (bps). The top speed of most dialup connections in the downlink direction (towards you) is 56 kilobits per second (a kilobit is a thousand bits).
Basic DSL (the broadband you get on your phone line) usually has a downlink speed (synonymous with the colloquial usage of bandwidth) of 768kbps (kilobits per second) but an uplink (from you) speed of only 128kbps.
Cable service these days often offers at least 3 megabits per second (a megabit is a million bits) down and 1.5 mbps (megabits per second) up.
Is a bit the same as a byte? (told you this wasn’t for nerds). No; a byte consists of eight bits. File sizes are usually measured in bytes so an 8 megabyte file has 64 megabits in it. In a perfect world (which assuredly doesn’t exist), it would take 64 seconds (plus a few more for some control bits) to download this 8 megabyte file over a connection which has 1 mbps of downlink bandwidth.
Then what DOES it mean that I pay for an x megabit connection if I can’t count on it to download x millions of bits per second? What AM I paying for? Why can’t I count on downloading at rated speed? Isn’t there any kind of “truth in bandwidth”? Starting with the last question first, no, there is no truth in bandwidth. Most vendors describe the MAXIMUM capacity of the link between you and them (not between you and the Internet) when they quote bandwidth. The fine print almost always says that experience will vary.
Why? First of all, many Internet links are actually shared even between the subscriber and the Internet Service Provider (ISP). Cable is shared; DSL is not; dialup is not; some radio connections are shared and others aren’t; satellite is shared on the downlink side only. So, if every user who is connected is trying to run at maximum speed on a party line, no one is gonna achieve maximum speed. If everyone gets on the freeway at once, no one gets to drive at the speed limit. Note that even when connections are nominally not shared like DSL, there are technical reasons why too many connections at once can still degrade service through various types of interference.
Second, even if you are not sharing the connection between your computer and your ISP, you are usually accessing web sites located somewhere on the Internet other than on the network of your own ISP. Those web sites are connected to the Internet through their own ISPs. And then there are intermediate ISPs (the Internet backbone) between your ISP and the ISP of the website you’re trying to download from. If your ISP has, for example, exactly one thousand customers each with one mbps of download capacity, the ISP’s connections to the rest of the Internet may total only 25 mbps (or less) even though all of you downloading together could theoretically use a gbps (a gigabit is a thousand million or one billion bits).
This isn’t fraud; it’s the way the Internet is built. The highways system wouldn’t work if every driveway disgorged a constant stream of cars. The phone system can’t handle more than a fraction of the phones being in use at once; it gives busy signals. The Internet doesn’t give busy signals; it just gets slow.
The third reason you may not get the speed you imagined you paid for is that the computer which runs the website you’re accessing is too busy to feed you data as fast as you think you ought to get it. Of course that computer also faces bottlenecks in any shared connections it has and on its ISP’s connection to the Internet backbone.
Fourth major reason for below-rated performance is the portion of the Internet backbone between your ISP and the ISP of your data provider may be congested. The Internet tends to route around congestion but TENDS is the operative word.
And a fifth reason, in case you still want one, may be the quality of your connection or some intermediate connection. A poor quality connection or even a very congested one will lose data. Most uses of the Internet have a way to request the retransmission of lost data but retransmission takes time and reduces the effective bandwidth available to you.
More detail on bandwidth here.