Mark Seams. When you are ready to unwrap your mesh to create a UV map, you must first of course select your seams. That is where this tool comes in. You can select edges with the LMB, edge loops with SHIFT+LMB and deselect with CTRL+LMB.
Edge loops. This tool is no different then holding down SHIFT+LMB with the “Mark seams” tool. The difference here is that you are not required to hold down the SHIFT key to select your desired edge loops.
Clear clusters. Clears all UV clusters (also called UV islands) and seams.
Clear seams. Clears only the seams of the current mesh.
Auto seams. This function is mostly self explanatory. It will essentially create seams automatically with a “best guess” on what edges to select as the seams.
Pack UV. This will pack all unwrapped UV clusters onto the UV map. When this operation is
complete, you can then edit the islands on the UV map. At this point you can perform many essential functions which require a UV map.
Upd. Islands. This command updates UV islands in the preview window after any adjustments that you've performed (marking seam, deleting seam, etc.)
Save/Load. You can also save and load seam and cluster information of the current scene so it can be edited later. Note that this does NOT export or import a UV map.
Further advanced tools for editing your UV maps directly reside in the “Selected” portion of the of the retopology tools. These tools all require a selection in the UV Preview window. To select a cluster (or island) simply click on one with the LMB.
Rotate CW. This will rotate the selected island clock-wise.
Rotate CCW. This will rotate the selected island counter clockwise.
Flip U. This will flip the U of the UV map.
Flip U. This will flip the V of the UV map.
Relax. Applies a light smoothing across the selected cluster (or island) to relieve stretching or pinching.