Uncomformities are any geological feature in which there is a transition between rock layers that marks a gap in the sequence---intermediate layers have somehow been removed, or rocks produced in different eras have somehow been pushed together.
An angular unconformity is when one group of sediments has been titled by tectonic forces so that it is tilted at an angle with Earth's gravity, and then more layers of sediment were produced on top of it. Instead of the usual geological column of layer after layer, we have two sets of layers that abut against each other at an angle.
A disconformity is when sediment layers were eroded away and then new sediment was formed on top of them, so that there are intermediate layers effectively "missing" that would ordinarily be present.
Finally, a nonconformity is when igneous or metamorphic rocks are layered over by sediments; these indicate extremely long periods of erosion where all the previous sedimentary layers were removed to expose the igneous or metamorphic rock below.
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