What is the difference between GB and GiB?

There are actually two differences.

The first of them, as noted in the accepted answer, is that the terms "kilobyte", "megabyte" and "gigabyte" according to the recommendation IEC should mean 1000, 1,000,000 and 1,000,000,000 bytes, respectively. In contrast, the terms "kibibyte", "mebibyte", and "gibibyte" should mean 1024, 10242 = 1,048,576, and 10243 = 1,073,741,824 bytes.

In reality (and this is the second difference), the " bi " variants are practically not used, and in in everyday communication, kilobytes, megabytes, and gigabytes most often (but not always) mean 1024, 10242, and 10243 bytes. This creates confusion, but terms like "kibibyte" are so discordant that you have to live with this confusion.

However, manufacturers of disk drives and RAM chips often use traditional units ("gigabytes") to refer to the capacity of their devices, using them in the sense of the IEC recommendation (that is, they have 1 GB = 1,000,000,000 byte). Critics argue that this is not so much to support standardization as for marketing reasons.

GiB (gibibyte) 73,741,824 bytes more than GB (gigabytes). Gibibyte was designed to replace the term gigabyte in those areas of computer science in which it meant 1,073,741,824 bytes, which contradicts the SI definition for the prefix giga- (109 = 1,000,000,000, billion).