What is Ghotra? How it differs from caste?

@Gulshan, in the HIndu religion, it is believed that all the communities have descended from Rishis or Munis from ancient times. For eg, my gotra is Mudgal, Mudgal is the name of one of the Rishis. Hence it is belived that all the people whose gotra is Mudgal, has descended from Mudgal Rishi and are therefore of one blood. It is due to this fact, the same-gotra people are considered as brothers and sisters and hence the opposition to same gotra marriages.

But considering that centuries have passed since the actual families started by these rishis and it is bound to have happened that the descendents married within their own communities. Hence the gene pool and blood pool cannot have remained pure for any gotra. Therefore, this belief is NOT at all relevant now and only a cause for horrible incidents of honour killing.