The N-Forms were machines built and programmed by the Patrexes Chapter of the Time Lords for use against the Great Vampires. After the war ended, their central intelligence core was downloaded and they were shut down.
They were machines that were programmed to destroy their enemies and were quite capable of destroying a planet to accomplish their goal. They were capable of attaching themselves to objects on a molecular level and opening tiny dimensional rifts inside the scarred brains of people that ingested the compound to which the N-Form had attached itself. This allowed them to emerge from thousands of points rather than from a single large, potentially continuum destroying rift. This was done through the N-Forms ability to slip between dimensions allowing their extrusions to kill their enemies.
They were only capable of being shut down by interacting with them through an engram. It was also possible to attempt to reason with an N-form, though they were usually single-minded in their tasks. (PROSE: Damaged Goods)
An N-Form awakened by the Brotherhood of the Immanent Flesh took the form of Mei Feng and chased Roz Forrester across Earth in 2982. Roz flattened the N-Form with dwarf star alloy. (PROSE: So Vile a Sin)
During the Last Great Time War, the War Council sent out an activation signal, reactivating every N-Form laying dormant in history. (AUDIO: Desperate Measures) The Seventh Doctor encountered one such N-Form which had bonded with the corpse of Simon Jenkins. (AUDIO: Damaged Goods)