RUF is the Porsche tuner. Because the Porsche license is owned by EA (Electronic Arts), Polyphony couldn't use Porsches in the game. However, EA doesn't own the license for RUF (the Porsche tuner) so RUF tuned Porsches are available.
Example: Say EA owned the Viper license (so there would be no Vipers in the GT Series) but didn't own the Hennessey license (a Viper tuner) they would be able to use Hennessey vipers, like the Venom 800TT, etc.
BTW, I don't know what Tommy Kaira does, I reckon there some car manufacturer that no-one knows about