Apache Solr is an open-source search platform built on Apache Lucene. It is a highly scalable and fault-tolerant full-text search server that provides distributed indexing, replication, and load-balanced querying, making it ideal for use in applications requiring large-scale search capabilities.

Solr supports a wide range of features, including advanced full-text searching, faceted search, result highlighting, spell checking, and more. It can index structured, semi-structured, and unstructured data in various formats such as XML, JSON, and CSV. Solr also offers powerful query language and extensibility options, allowing developers to customize and optimize their search applications.

Some key benefits of using Solr as a full-text search server include:

1. Scalability: Solr can handle large volumes of data and accommodate high query loads with its distributed architecture.

2. Fault tolerance: Solr provides automatic failover and disaster recovery capabilities to ensure uninterrupted search operations.

3. High performance: Solr is designed for high-speed, real-time indexing and querying, allowing for fast and efficient search results.

4. Flexibility: Solr can be easily integrated with other systems and applications using its JSON and XML APIs, making it versatile for a variety of use cases.

5. Extensibility: Solr offers a range of plugins and extensions for further customization and enhancement of its features and functionality.

Overall, Solr is a powerful and reliable full-text search server that can be used to build robust search applications for a wide range of industries and use cases.