Rahu (North Node of Moon) has the nature of a serpent. It poisons you to the illusions of this world. It is our desires that keep us incarnating here.
Ultimately the disillusionment leads to the enlightenment of Ketu (South Node of Moon), who detaches and renounces the world for spiritual enlightenment. The nodes are the most powerful of all the planets, for they cause the eclipses of the luminaries, the Sun and Moon. The nodes block out the light or consciousness.
Co-owned sign and exalted debilitated sign for rahu and ketu are stated different in varied vedic scriptures. However I am giving here what is stated in Parashari Hora and in which I believe too.
Rahu owns Aquarius (kumbh) and Ketu owns Scorpio (vrischika), rahu mooltrikona sign is Gemini (mithun) ketu mooltrikona sign is sagitarius (dhanu).
Rahu gets exalted in taurus and debilitated in scropio, ketu gets exalted in scorpio and debilitated in taurus.
Rahu and ketu gives best result in 3rd, 6th and 11th house.However ketu being mokshkarka also works good in 12th house making the native spiritual and leading a king like life.
Rahu and Ketu gives result according to their depositor, sign, house in which it is placed, aspects and co-junctions.
It is also said that Rahu and Ketu amplifies the result of planet they sit with. It means when co-joins benefic planet, they increases its beneficial tendency to great extent.
Rahu is separative in nature and ketu being inclined towards attachments spiritually. Suppose rahu is sitting in 7th house or co-joins 7th lord makes separative yoga with wife (Note: according to the degree of co-juntions with 7th lord, more closer makes more separative yoga) but ketu does opposite to rahu in these positions.
They tends opposite qualities, for example conjunction of rahu + moon makes the native harsh, rude and not to belive on religion on the other hand conjunction of ketu + moon makes one spiritual and fickle minded.