Mercury represents thought and communication: how you reason, learn, speak, and exchange information. The sign it occupies gives a style of mind — quick and curious, methodical and precise, or any other nuance depending on the sign involved.
Mercury is never very far from the Sun in the sky (at most ~28°): that is why it is always found in the same sign as the Sun, or in one of the two neighboring signs.
Domicile: Gemini and Virgo. Detriment: Sagittarius and Pisces. Fall: Pisces. Mercury has no fixed sect (diurnal or nocturnal): its nature follows its position relative to the Sun — "oriental" (rising before it) it behaves as a diurnal planet, "occidental" (rising after) as a nocturnal planet.
Mercury finds its joy in the 1st house (identity itself, the way of presenting oneself).
A Mercury retrograde (about 3 times a year, ~3 weeks each) is traditionally read as a period when reflection takes precedence over quick action — a time to reread, revise, and go back rather than move forward.