An exabyte is a very large unit of storage space, 1018 bytes. It is a billion gigabytes, a million terabytes, or a thousand petabytes. The term "exabyte" tends to be used in a way similar to the way the term "terawatt" is used - to quantify humanity-wide measures, such as the amount of information humanity generates in a full year. Practical use of the word exabyte started in the late 90s, as the Internet was ballooning in size and studies were being done on historical measures of information production.
One of the first estimates that used the term "exabyte" was a Berkeley study attempting to quantify the sum of human-produced knowledge, including all audio, video recordings and text/books, at the end of the 20th century. The value came in at about 12 exabytes. Due to the increased storage capacity of computers, we are able to store much more of the information we generate, but the availability of the storage space itself encourages us to create information much faster. A more recent study, completed in 2007 by the International Data Corporation, suggested that we produced 161 exabytes of data in 2006, though this takes into account duplicates. Ignoring duplicates, the figure is closer to 50 exabytes. We are currently creating data faster than we can store it, and as a result have an incentive to keep developing digital storage technologies.
One popular use of the term exabyte involves the quantification of all human words ever spoken. Converted into text, this is estimated at about 5 exabytes of data. If digitized as 16 Khz 16-bit audio, it is estimated that the value would be much higher, around 42,000 exabytes. On the popular television series Star Trek, the character Data was said to have a storage capacity of 0.1 exabytes. An actual human brain may in fact have a storage capacity significantly lower than this, perhaps even by several orders of magnitude.
Today, buying an exabyte of storage space would cost roughly $500 million US Dollars (USD). The storage capacity of all the world's computers is in the dozens of exabytes. But the cost of storage space is cut in half almost every year, so the amount of disk space we can afford also doubles. If the current rate of price decline continues, by 2025 we'll be able to buy an exabyte of disk space for only $4,000 USD. We may be tempted to say that this is more than enough disk space for anyone, but humanity's history of voracious data collecting and generating suggests that there may be some files that even an exabyte drive can't contain.