I have seen a lot of confusion among people when it comes to the use of KB kB Mb MB etc. I hope this clears out your confusions.

FIRSTLY, there is a lot of difference between a UPPERCASE ‘K’ and a lowercase ‘k’. The same applies with the ‘B’ and ‘b’. So, keep in mind the case.

Now, what’s the difference between these letters: -

K = 1,024 (multiplier)
k = 1,000 (multiplier)
B = Bytes (unit)
b = bits (unit)

Generally, we see combinations of K,k or M,m with B,b.
Here, the B represents Bytes(generally used in storage) and the b represents bits(used in data transfer speed).
The K,k stand for Kilo, and M,m stands for Mega.
And,
1 B (Byte) = 8 b (bits)

The K,k and M,m are the multipliers.
K stands for *1,024 and k stands for *1,000 (Same for M,m)

So, 1 MB(MegaByte)= 1,024 KB (KiloByte)
AND, 1 mbps(megabit per second)= 1,000 kbps (kilobit per sec)

Now lets see these units separately in terms of Storage, and Data transfer speed.

1) Storage.

File size (how big a file is on your computer) is normally measured in units of “kilobytes,” “megabytes,” and “gigabytes.” In this computing the usage of an uppercase unit represents a multiplier of 1,024. for example

1 KB (one KiloByte) = 1,024 Bytes
1 MB (one MegaByte) = 1,024 KB
1 GB (one GigaByte) = 1,024 MB

Hence,
1 GigaByte = 1,024 MegaByte = 1,024* 1,024 KiloBytes = 1,024* 1,024* 1,024 Bytes

2) Data transfer speed.

Data transfer speed is measured in terms of kb or kilobits. Data transfer speed is sometimes discussed using Bytes, but this usage is ambiguous and discouraged, as data transfer speeds are measured using “bit-rate” (bits per second).

1 kbps (one kilobit per second) = 1,000 bits per second
1 mbps (one megabit per second) = 1,000 kbps (i.e. one million bits per second)
1 gbps (one gigabit per second) = 1,000 mbps (i.e. one billion bits per second)

Also note these values: -

(IN TERMS OF BYTES):-
K (Kilo) = 10^3 = 1,000
M (Mega) = 10^6 = 1,000,000
G (Giga) = 10^9 = 1,000,000,000
T (Tera) = 10^12= 1,000,000,000,000
P (Peta) = 10^15= 1,000,000,000,000,000

(IN TERMS OF BITS):-
K (Kilo) = 2^10 = 1,024
M (Mega) = 2^20 = 1,048,576
G (Giga) = 2^30 = 1,073,741,824
T (Tera) = 2^40 = 1,099,511,627,776
P (Peta) = 2^50 = 1,125,899,906,842,624

-It is also seen that, instead of the uppercase ie. K, one might see a Ki. For example, instead of a KB, KiB is also used. They both have the same values.

-KiloByte is also abbreviated as KB, kB, Kbyte, kbyte, or very informally, K or k.