“Big data” is actually just a lot of data. It is a group of obstacles and technologies associated to the availability of excessively large amount of data that businesses need to connect and figure out. The main reason why the sector is hot today is that the data and tools have attained a critical mass. This appeared in parallel with years of study effort that has convinced management that they must do something with their data wealth. While it’s even more of a marketing term than anything, the implication is generally that you have so much information that you can’t analyze all of the data at one time because the amount of memory (RAM) it would consume to hold the data in memory to analyze and process it is higher than the amount of available memory.
This implies that analyses usually have to be done on arbitrary segments of data, which permits models to be assembled to compare against different parts of the data.