Hi Marnie,

Since has remained unanswered for over a week I'll have a go at explaining the difference.

Although they both nominally mean 'While', 'pendant que' is more of a temporal conjunction indicating that indicates the time frame of an action during which something else happens.

Pendant que j'étais en France j'ai visité la tour Eiffel. - While I was in France I visited the Eiffel Tower.

Tandis que signifies a contrast and can often be translated as whereas.

Paul est un travailleur acharné tandis que son frère est un bon à rien. - Paul is a hardworking man while (whereas) his brother is a wastrel.

Hope this helps,

Tom

Salut Tom!  That makes sense...and resound in my echo-chamber of previously known & spoken French!

Merci!