I would say, that simply speaking, Dogmas are those truths that the Church has decided on definitively. Some examples would be: The Assumption of Mary, Transubstantiation (the name for the change that takes place when the bread and wine become the Body and Blood of Our Savior), and the Infallibility of the Pope when he speaks “ex-cathedra”.
Doctrines, while still truths of the faith, are of a somewhat lesser scale, only in the regards that they have not been defined solemnly.
Usually, the reason a truth is declared a dogma, is because there has arisen a dispute over it and so it needs to be clarified.
Hope that helps!