A rashi is a 30° slice of the zodiac, and there are twelve of them. That much is shared with Western astrology. Where the two part company is the question of where the zodiac starts, and the answer is the single largest technical difference between them.
Western astrology uses the tropical zodiac: 0° Aries is defined as the position of the Sun at the March equinox. Jyotish uses the sidereal zodiac: 0° Aries is fixed against the background stars. Because the Earth's axis precesses — a slow wobble completing a circuit in about 25,800 years — these two starting points drift apart by roughly 50 arcseconds a year. They coincided some time in the early centuries CE and are now about 24° apart, which is most of a whole sign.
The practical effect is that most people have a different Sun sign in the two systems, usually the one before. Someone told all their life that they are a Leo will often find their Jyotish Sun in Cancer. Neither system is making an arithmetic mistake; they are measuring from different origins and are internally consistent about it.
A rashi carries an element (fire, earth, air, water), a mode (movable, fixed, dual), and a ruling graha. Those three properties, plus which graha is exalted or debilitated there, are most of what a beginner needs. What a rashi does not carry is a life area — that is the job of the bhava, and confusing the two is the commonest early mistake.