The Samanids replaced the Tahiridsand were the second native dynasty to rule Persia and transoxania not the first. The family was distinguished in the court of the caliphate and in the politics of the eastern part of the empire however their most important legacy was their islamisation of the seljuq, which was their subsequent demise. The started but did not finish the persification of the culture of eastern part of the caliphate. It was the seljuqs and Ghaznavids who really finishsed the persification. The Samanids were still very fond of Arabic culture since the beginning.
Here is an article about the Samanids:
As for what happened to them, well, unfortunately they were the only civil state in the region and their resources were much limited. Their oponents, the Karakhanids were a tribal based empire and it had the support of the majority of non Iranians, who were mostly Turks. In those days it was really difficult to defeat such steppe empires with regular troops and the Samanids failed to take precaution and were invaded and divided.
Al-Jassas