A node in Cassandra is used to store data. That is it. Each node is present inside a cluster but is independent, and interconnected at the same time. Each node stores replicates the data, and this helps in offering no point of failure in Cassandra. Because if one node is down, another node is used to perform the operations, thus offering scalability and reliability to its customers.
If you are interested in getting trained and certified in Apache Cassandra, check out the Cassandra training course from Intellipaat. Also, have a look at the following YouTube video on Apache Cassandra Training, made by industry experts.