Apache Ignite is

the in-memory computing platform

that is durable, strongly consistent, and highly available
with powerful SQL, key-value and processing APIs

So, in summary, Ignite looks like a distributed data storage that can work both, in-memory and on-disk, and provides SQL, key-value and processing APIs to the data. Sounds simple enough. However, to get a complete picture, perhaps it is better to define Ignite by answering several "Is Ignite a ...?" questions:

Both. Native persistence in Ignite can be turned on and off. This allows Ignite to store data sets bigger than can fit in the available memory. Essentially, the smaller operational data sets can be stored in-memory only, and larger data sets that do not fit in memory can be stored on disk, using memory as a caching layer for better performance.

Yes. Even though Ignite durable memory works well in-memory and on-disk, the disk persistence can be disabled and Ignite can act as a pure distributed in-memory database, with support for SQL and distributed joins.

Yes. Ignite is a full-featured data grid, which can be used either in pure in-memory mode or with Ignite native persistence. It can also automatically integrate with any 3rd party databases, including any RDBMS or NoSQL stores.

Yes. Data in Ignite is either partitioned or replicated across a cluster of multiple nodes. This provides scalability and adds resiliency to the system. Ignite automatically controls how data is partitioned, however, users can plugin their own distribution (affinity) functions and collocate various pieces of data together for efficiency.

Not fully. Although Ignite aims to behave like any other relational SQL database, there are differences in how Ignite handles constraints and indexes. Ignite supports primary and secondary indexes, however, the uniqueness can only be enforced for the primary indexes. Ignite also does not support foreign key constraints.

Essentially, Ignite purposely does not support any constraints that would entail a cluster broadcast message for each update and significantly hurt performance and scalability of the system.

Not fully. ACID Transactions are supported, but only at key-value API level. Ignite also supports cross-partition transactions, which means that transactions can span keys residing in different partitions on different servers. At SQL level Ignite supports atomic, but not yet transactional consistency. Ignite community plans to implement SQL transactions in version 2.4.

Yes. Ignite provides a feature rich key-value API, that is JCache (JSR-107) compliant and supports Java, C++, and .NET.

You can find out more about Ignite by visiting the freshly redesigned Ignite website.