The difference Between Rashi and Lagna is a bit tricky to understand but it is simple. Rashi is the sign where the Moon is posited at the time of birth. Moon will stay in a sign for around 2 1/2 days. Lagnam is the rising sign at the time of birth.
Lagnam is determined with the help of the time of birth and place of birth. One Earth day is 24 hrs and during this 360 degree of turn, we will face or see all the 12 zodiac signs within the interval of ~ 1.4 to 2.6 hrs
So i will give an example:
A Boy is born on April 17th, 1985, at 4.30 AM in Delhi – India. The Sunrise time indicated during that day is at 5.58 AM. At the Eastern horizon exactly by 5.48 AM, Sun will appear in the sign of Aries and will stay there till 7.21 AM and then move to the next sign of Taurus. (appearance of Sun at the Horizon will change according to the locations). Since the person is born 1 hr 28 mins before Sunrise, His Lagnam or Ascendant will be Pisces.
The rising sign is the person’s worldly personality. Everything related to his health, looks, character, confidence, etc is denoted by the Ascendant or Lagna, or in simple terms, Lagna is ‘you’ in this physical world. And so Astrologically, the planet which is representing Lagna lord will become the most powerful and beneficial one in the birth chart. Lagna or Ascendant sign becomes the 1st house and primary reference point for a birth chart
Rashi is the Moon sign and it represents a person’s Mind which is also an important matter when it comes to living our physical life in this world. So Rashi becomes the second important reference point in a birth chart. Once after birth, the planets in transit can influence the mind of a person. This is the reason we take Rashi as the reference point while analyzing the transits.