NoSQL databases, or "Not Only SQL" databases, are a class of database systems that diverge from the traditional relational database management systems (RDBMS) model. While RDBMS systems organize data into structured tables with predefined schemas and use SQL (Structured Query Language) for data manipulation and querying, NoSQL databases offer a more flexible approach to data storage and retrieval.

Here are some key characteristics of NoSQL databases :

Non-relational Data Model : NoSQL databases do not adhere to the tabular structure of RDBMS. Instead, they use various data models such as key-value, document, column-family, or graph to organize data based on the specific requirements of the application.

Schemaless Design : Unlike RDBMS, which enforce a rigid schema where each row must adhere to a predefined structure, NoSQL databases typically allow for dynamic schema design. This means that different rows or documents in the same collection can have different sets of attributes.

Scalability : NoSQL databases are designed to scale horizontally to handle large volumes of data and high throughput. They distribute data across multiple servers, allowing them to accommodate growing workloads by adding more hardware resources.
High Performance : Many NoSQL databases are optimized for high performance and low-latency operations. They often sacrifice some features of traditional databases, such as complex transactions and joins, in favor of speed and scalability.

Flexibility : NoSQL databases can handle a wide variety of data types, including structured, semi-structured, and unstructured data. This flexibility makes them well-suited for modern applications that deal with diverse data sources and formats.

CAP Theorem : NoSQL databases are often designed with consideration for the CAP theorem, which states that it is impossible for a distributed system to simultaneously provide all three of the following guarantees: consistency, availability, and partition tolerance. NoSQL databases typically prioritize either consistency and partition tolerance (CP systems) or availability and partition tolerance (AP systems), depending on the specific use case.