Bill Burnham writes:
To understand the forest you need three main things: persistent search, state, and metadata. Persistent search is simply a search query that is constantly running. State is information about the state (what time is it, what things are connected to what, etc.) of the web each time a query is run. And metadata is information that is derived from the set of search results returned for a particular query (how many results, what type of results, who authored the results, statistical analysis of elements within the results, etc.).
Marrying these three items together adds two key dimensions to search: time and context. Time is an incredibly important dimension because once you can compare things over time, you can determine change and the rate of change.