Memcached is a key-value store used as a cache. It is designed to be simple and is, therefore, limited in some ways. These limitations can also be looked at as features because they make Memcached transparent.
Keys in Memcached are strings, and they are limited to 250 characters. Values can be any type, but they are limited to 1 MB by default. Keys also have an expiration date or time to live (TTL). However, this should not be relied on, as the least recently used (LRU) algorithm may remove expired keys before they are accessed. Memcached is a good choice for caching expensive queries, but it should not be relied on for persistent or durable storage. Always build your application around Memcached not having what you want. Plan for the worse, hope for the best.