Hey there!
First of all, before we get into this, it's important to note that there are two types of crust: oceanic crust, which is located under the oceans of the world and is much thinner than the other type of crust, which is continental crust, located on the continents and is much thicker.
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The first type of boundary is a convergent boundary.
At a convergent boundary, the plates are moving toward each other and crashing into each other, or colliding.
At a convergent boundary between one continental plate and one oceanic plate (continent-ocean) and a boundary between two oceanic plates (ocean-ocean), one plate will subduct under the other, always the oceanic plate in a continent-ocean boundary. The crust that subducts under the other melts into magma.
In the case of two continental crust pieces converging (continent-continent), they don't really subduct under each other, they just shove up into each other.
Main effects of a convergent boundary include volcanoes, mountain ranges, and earthquakes/seismic activity.
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The second type of boundary is a divergent boundary.
At a divergent boundary, the plates are moving away from each other.
At an oceanic divergent boundary, the separating allows a fissure to be opened and, in many cases, it acts like a cut that's healing. New magma comes up, which creates new crust.
At a continental divergent boundary, it pulls apart, creating faults and earthquakes. This forms a rift. Water flows into the rift as well. If you've ever heard of the Great Rift Valley in East Africa, that's a great example of this type of boundary.
Sometimes there is volcanic activity along rifts made by divergent boundaries.
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The third type of boundary is a transform boundary.
At a transform boundary, the plates are sliding past each other.
One main effect of a transform boundary is earthquakes. Tsunamis are also an issue. Tsunamis are sometimes caused by an earthquake.
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Hope this helps!
