Buffy,
MSA is Minimum Safe Altitude, and gives you the company/state approved vertical clearance over all terrain and obstacles in the specified area. If the area is between two en-route points, then a bandwidth 20nm either side of track is applied.
SSA is Sector Safe Altitude, applicable as Exeng says to a quadrant of an instrument approach chart, and gives company/state approved clearance as above within that quadrant. The range for the quadrant is usually 25nm, with a 5nm buffer applied both outside, and into adjacent quadrants.
Approved vertical clearances are based on the elevation of the obstacle, and are of the order 1000ft for obstacles up to and including 5000ft, and 2000ft above that.
Some companies advise increasing the MSA by an arbitrary amount if the wind at the MSA is above a certain strength (increase MSA by 2000 feet if 50kts or greater at the MSA) or the atmosphere significantly colder than ISA (4% of height per 10 degree C below ISA comes to mind, but I can't get at the books right now to confirm.)
Stay safe!
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Disappears in a cloud of black ink... should use some of it to spell correctly in the first place, then wouldn't need to edit!
[This message has been edited by Sick Squid (edited 17 November 2000).]