Metamorphic rocks have had their structure changed and can also have their chemical make up changed because of being reheated to high temperature and subjected to high pressure.
The high temperature doesn't cause the rock to melt, but to recrystallize into new forms; if the rocks melted they would become igneous rocks and not metamorphic rocks.
There is also a contact metamorphic form caused by contact with high temperature magma that flows out of a volcano and comes in contact with the country rocks that are present.
These metamorphic changes can occur to Igneous, Metamorphic or Sedimentary Rocks.
Here are a few of the changes that take place in rocks; Granite becomes Gneiss, Sandstone becomes Quartzite, Slate becomes Schist, Limestone becomes Marble.