This is no longer a part of the World of Warcraft API. It was removed in patch 3.3.

Find all players who are looking for a group for a specified objective

LFGQuery(category, objective[, class])

TakesEdit

Number category

Must be the index of one of the values retrieved with GetLFGTypes()

Number objective

Must be the index of one of the values retrieved with GetLFGTypeEntries(category)

Number class (optional)

You can probably specify which character class you are searching for (priest, warrior, druid, etc.), but exact usage is unknown

ReturnsEdit

Nothing

EventsEdit

Triggers UPDATE_LFG_LIST

Triggered as soon as query results become available (works, tested on Live client - Dec 27, 2006)

DetailsEdit

Use GetNumLFGResults() to retrieve the number of results and then iterate over them with GetLFGResults(index).

Reqires a hardware event for each call to the API, so it should be called from related script handlers such as OnClick. (tested on Live client - Dec 28, 2008)