A bell nozzle will work optimally only at one altitude (pressure). They pick that location as the mean of where it has to work and design it to be okay over the range.
The aerospike doesn't have this limit.
Fuel is pumped just under nozzle surface to cool it (as is done in other nozzles) so it enters combustion chamber hotter.
Here are some links:
- http://en.wikipedia.org/wiki/Aerospike_engine
- http://www.nasa.gov/centers/marshall/news/background/facts/aerospike.html
- http://www.astronautix.com/stages/satt250k.htm
- http://www.aerospaceweb.org/design/aerospike/main.shtml