Hi,

Development set is a part of the initial data (usually a percentage from it)  used to create (to build) the actual model. Validation set, is a different part of the data used to test the accuracy of the model. The validation set is not 'seen' during the model development. You can see it as you want to develop a model on a data set (development set), and then you want to see how well it performs on new data, which you have not seen so far (validation).

Hope it makes sense.

Miki