According to the novel canon, the Formics attacked Earth a few generations before the novel begins. They attempted to colonise the planet and were barely fought off by an Earth soldier known as Mazer Rackham. The first book in the series, Ender's Game, largely stems from the human quest to defend themselves from this species, although the Formics ultimately turn out as victims, with the first attack being an accident due to differing biology.
The term "Formic" is derived from formica, the Latin word for ant; whereas "bugger" is a pejorative used by humans; yet it was not until 1999's Ender's Shadow that the term 'Formic' was first used, interchangeably with 'Bugger'. Later books used 'Formic' almost exclusively, as the more 'scientific' term. This leads to odd scenarios in the continuity of the books, such as Valentine referring to them as "Buggers" in Ender's Game, chronologically next as "Formics" in Ender in Exile, and again as "Buggers" in Speaker for the Dead and Xenocide. The feature film adaptation of Ender's Game uses "Formics" exclusively.
The Formic species consists of hive-minded colonies directed by queens. While often described as "insectoid", the Formics are warm-blooded, and they respire and perspire. If a queen dies, all the workers under her control lose their intelligence and ability to function immediately; but in Xenocide, implications exist that 'workers' can escape the influence of a queen. The Formic race is revealed to be trimorphic in Shadows in Flight. Drones are much smaller and depend on a Hive Queen for survival, and their bodies are shaped to spend their lives clinging to her, until upon her death, they take flight to seek out a new queen. Drones are capable of individual thought and action as well as mind-to-mind communication, more limited than that of a queen; whereas queens communicate instantaneously and can even do so with other species. Formics live in vast underground colonies, usually without light, informing the assumption that Formics make use of sensory apparatus outside the range of the electromagnetic spectrum visible to humans. In the first novel they have artificial lighting; whereas in Xenocide, Ender claims they rely on heat signature.