Cassandra is a type of database platform designed to store massive amounts of data across numerous machines. It is one of several kinds of 'NoSQL databases,' meaning that it doesn't rely exclusively on SQL to manipulate the data it stores ("not only SQL"). Cassandra is scalable, meaning that it can grow its storage capacities to meet the demand of new data. It is also designed to provide high availability for data. Additionally, Cassandra provides a flexible schema design, meaning that it can store new data with old data that may not resemble each other in terms of the attributes they have.
Cassandra is similar to Hadoop in that they both address ways to store massive amounts of data. Cassandra differs from Hadoop, however, in that it is able to complete computations much faster than Hadoop, and consequently is better suited for organizations that need fast access to information.