redis是一个key-value存储系统。和Memcached类似，它支持存储的value类型相对更多，包括string(字符串)、list(链表)、set(集合)、zset(sorted
set --有序集合)和hash（哈希类型）。这些数据类型都支持push/pop、add/remove及取交集并集和差集及更丰富的操作，而且这些操作都是原子性的。在此基础上，redis支持各种不同方式的排序。与memcached一样，为了保证效率，数据都是缓存在内存中。区别的是redis会周期性的把更新的数据写入磁盘或者把修改操作写入追加的记录文件，并且在此基础上实现了master-slave(主从)同步。

Redis 是一个高性能的key-value数据库。 redis的出现，很大程度补偿了memcached这类key/value存储的不足，在部 分场合可以对关系数据库起到很好的补充作用。它提供了Python，Ruby，Erlang，PHP客户端，使用很方便。

Redis支持主从同步。数据可以从主服务器向任意数量的从服务器上同步，从服务器可以是关联其他从服务器的主服务器。这使得Redis可执行单层树复制。从盘可以有意无意的对数据进行写操作。由于完全实现了发布/订阅机制，使得从数据库在任何地方同步树时，可订阅一个频道并接收主服务器完整的消息发布记录。同步对读取操作的可扩展性和数据冗余很有帮助。

官方英文原文介绍Redis

Introduction to Redis

Redis is an open source, BSD licensed, advanced key-value store.
It is often referred to as a data structure server since
keys can contain strings, hashes, lists, sets and sorted
sets.

You can run atomic operations on these types, like appending
to a string; incrementing
the value in a hash; pushing
to a list; computing
set intersection, union and difference;
or getting
the member with highest ranking in a sorted set.

In order to achieve its outstanding performance, Redis works with an in-memory
dataset. Depending on your use case, you can persist it either by dumping
the dataset to disk every once in a while, or by appending
each command to a log.

Redis also supports trivial-to-setup master-slave
replication, with very fast non-blocking first synchronization, auto-reconnection on net split and so forth.

Other features include Transactions, Pub/Sub, Lua
scripting, Keys
with a limited time-to-live, and configuration settings to make Redis behave like a cache.

You can use Redis from most
programming languages out there.

Redis is written in ANSI C and works in most POSIX systems
like Linux, *BSD, OS X without external dependencies. Linux and OSX are the two operating systems where Redis is developed and more tested, and we recommend
using Linux for deploying. Redis may work in Solaris-derived systems like SmartOS, but the support is best
effort. There is no official support for Windows builds, but Microsoft develops and maintains a Win32-64
experimental version of Redis.

学习redis最好的资料就是官方网站的文档http://redis.io/