SQLite is an in-process library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. It's an embedded SQL database engine. Unlike most other SQL databases, SQLite does not include a separate server process. SQLite will read and write directly to ordinary disk files. It functions as a complete SQL database with multiple tables, indices, triggers, and views, all contained in a single disk file. The database file format is cross-platform - you can freely copy a database between 32-bit and 64-bit systems or between big-endian and little-endian architectures. These features make SQLite a popular choice as an Application File Format. SQLite database files are a recommended storage format by the US Library of Congress. Think of SQLite not as a replacement for Oracle but as a replacement for fopen()
SQLite is quite a compact library; for instance, with all features enabled, the library size can be less than 600 KB, depending on the target platform and compiler optimization settings. There is a tradeoff between memory usage and speed. SQLite generally runs faster the more memory you give it. Nevertheless, performance is usually quite good, even in low-memory environments. Depending on how it is used, SQLite can be faster than direct filesystem I/O.
SQLite is very carefully tested before every release and has a reputation for being very reliable. The majority of the source code is devoted to testing and verification processes. An automated test suite runs millions of test cases involving hundreds of millions of individual SQL statements and achieves 100% branch test coverage. SQLite responds gracefully to memory allocation failures and disk I/O errors. Transactions are ACID even if interrupted by system crashes or power failures. All of this is verified by automated tests using special test harnesses that simulate system failures. Of course, even with all this testing, there are still bugs. But unlike some similar projects (primarily commercial competitors), SQLite is open and honest about all bugs and provides bugs lists and minute-by-minute chronologies of code changes.