Network explanation: megabytes or megabits?

Gb is supposed to refer to bits and GB is supposed to refer to bytes. Bandwidth is always measured in bits per second but files on disk are measured in bytes.

Your best bet is to have the VPS company define the terms they are using so that there is agreement because the terms are often misunderstood or misused.

Network bandwidth is typically expressed in quantity of bits per unit of time -- e.g. 45Mb/sec (small b), or 45Mbit/sec. This expresses a rate of transfer.

The amount of data transferred is typically quoted in an absolute quantity of bytes moved - e.g. a 50MB (large B) file, or 50MBytes of data.

Most colocation providers sell bandwidth by transfer rate - You are allowed so many bits per second, and are either capped at that rate or allowed "burstable bandwidth" (with burstable bandwidth you are typically billed based on the 95th percentile of your usage -- use a lot of excess bandwidth, get a bigger bill).

Some providers sell by quantity of data transferred -- this is more common with shared web hosting companies. You can convert this to a rate via a rough approximation (multiply the quantity number by 8, then divide by the number of seconds in a billing period - 2592000 seconds is approximately one month (30 days)).
The caveat here is that the rate you calculate is pretty much meaningless: You could do zero traffic for 29 days, then shove all 50GB out on day 30, and as far as your provider is concerned you're within the limits of your utilization. Limiting yourself to a maximum of the rate you calculate minus a small margin for padding almost guarantees you won't exceed your transfer cap, but may hurt performance unnecessarily.

The reverse of that formula will give you a rough approximation of max quantity transferred for a given rate cap, which is possibly more useful, but bear in mind that providers that bill based on rate count every bit that goes over the wire (packet, protocol and payload), so the actual quantity of data (payload) you can move is somewhat lower than the raw number would lead you to believe.

Besides the obvious bit/byte confussion, and the 1000 vs. 1024 discrepancy, there's another little known issue with bandwidth:

• 64Kbps is usually 64000 bps (?)
• 1024Kbps can be 1024000 bps (??)
• 1Mbps is just as likely to be 1024000bps as 1000000bps or 1048576bps (!)
• I've seen 1Gbps that turned out to be 1048576000bps, not 1073741824bps nor 1000000000bps (!!!)

why?

That's because the original use for digital communications was the phone system, which used an 8bit ADC (analog/digital converter) at 8KHz (8000Hz), generating 64000 bits per second. Then, T1 lines aggregated 24 of these voice channels, creating a commonly used 1536000bps implementation (usually sold as 1.5Mbit, but 1.5*2^20 should be 1572864bps).

At the same time, it was easy to sell on individual channels increment, that is 64000bps at a time. Much later, many newer transmission technologies are defined not in bps, but on multiples of 64k channels. Confusingly, these multiplier number is usually a binary-round number, so sometimes an 8Mbps can be 128*64000=8192000bps instead of 2^23=8388608.

Standard for the industry is that speed is quoted in Gigabits/second while measured throughput is quoted in GigaBytes of throughput (how much data your server sent/received)

Files are measured in bytes.
So when file download is the most important thing the connection is used for, it may be expressed in bytes per month.
But connection bandwidth is more commonly expressed in bits, as there's no direct correlation between 8 bit groups and bytes of files transferred. ( some bits are used for error correction, and some protocols may use other than 8 bit words, or even variable bit rates )

As an end note, the case is meaningful, b is bits, and B is bytes, although I've seen that this convention is often misused.

I'm suprised no one mentioned the term Mebibits, although the way of counting was mentioned.
1 Mebibit = 2*32 bits, or 1048576bits which is equivalent to 1024 kibibits.

2000 GB (gigabytes) in a month. As the other fine answers indicate, that could mean zero for 29 days and 2000 for day 30, or 66.6 GB per day for 30 days straight. Which, if my math is correct, means 6.172 Mbps (megabits per second) continuously for 30 days.

The math:

2000 gigabytes * 8 bits per byte = 16000 gigabits (16 terabits!)

16000 gigabits / 30 days = 533.33333333... gigabits per day

533.33333... gigabits per day / 24 hours in a day = 22.2222... gigabits per hour

22.222... gigabits per hour / 60 minutes in an hour = .37037037... gigabits per minute

.37037037... gigabits per minute / 60 seconds in a minute = 6.172839506172839506e-3 gigabits per minute

.006172839506... gigabits per minute * 1000 megabits in a gigabit = 6.172839506 megabits per second

6.172839506 megabits per second * 1000 kilobits in a megabit = 6172.8 kilobits per second

So, you can consume somewhere between 6.172 Mbps continuously for 30 days and 16 Tbps for one second then zero for the remaining 30 days minus one second, or somewhere in between before incurring extra charges (assuming you can actually consume 16 Tbps in a second, which is not possible with the hardware a VPS will probably provide).

20Gbps and 2000GB per month are both measurements on the exact same yard stick (both measuring amount of data per unit of time) -- it is just that 20gbps is way faster than 2000GB per month.

20 gigabits per second vs. 2000 gigabytes per month.

pretty simple to convert :

2000 GB / month = 6.17 mbps

20 gbps = 6480 TB / month