These notes are borrowed from @neumark's posting to google groups:

• Tiddlyweb is a key-value store.
• A bag is a namespace. Permissions are defined on bags and apply to all key-value pairs within.
• A tiddler is a (Key, Value) pair, where the title of the tiddler is the key, and the remaining parts of the tiddler are the value part. Tiddler values have a set structure in order to ensure that metadata is stored in a uniform way (tags,fields,text).
• Recipes can be used to query tiddlyweb for tiddlers. Recipes are composed of a list of (optionally filtered) bags. Upon executing the recipe, tiddlyweb returns their union. Several bags may contain a tiddler with the same title: In order to resolve this collision, the value associated with the given key in the last bag of the recipe will be used. This leads to "[[recipe cascading|recipe cascade]]", which is analogous to the way javascript prototypical inheritance works.

See also TiddlyWebFundament.