How was the altamont concert different from the woodstock festival?

They were markedly different in a number of ways.

Explanation: Firstly Woodstock was a far bigger concert with dozens of acts over 3 days whereas Altamont was restricted to The Rolling Stones and a small number of acts in support e.g. Jefferson Airplane.
The major difference however is in what they came to represent. Woodstock was the high point of the Peace and Love counter-culture of the late 60's. There were arrests largely for narcotics offences and there were major logistical problems resulting in the concert becoming free.
However it passed largely peacefully and came to symbolise what could be achieved through a communal approach to such a major event.
Altamont was a final free concert given by the Rolling Stones in many ways trying to represent the ethos of Woodstock. However from the onset there were major problems.
It was very badly organised with hundreds of thousands having to walk to the speedway stadium where the concert was taking place.
Secondly on the advice of the Grateful Dead the Stones got Californian Hell's Angels to act as security. British Hell's Angels had done the same at the Hyde Park concert earlier in the year, but there Californian counterparts reflected a completely different culture.
They used extreme violence both on and off the stage culminating in the fatal stabbing of Meredith Hunter right in front of the stage. The whole episode and the mood and tension is very well captured in the motion picture Gimme Shelter.
If Woodstock was a high point of the counter culture then Altamont in many ways was not only a low point but reflected how this age ofoptimism was coming to an end and a darker culture was emerging. This was reflected not only at Altamont but also the Manson murders and the Vietnam war with the My Lai massacres.
This would eventually lead to the emergence of a New Right, Conservative Neo-Liberal culture reflected in politicians such as Reagan.