From Wikipedia: http://en.wikipedia.org/wiki/Zealotry I'll quote an interesting part here.
The Zealots objected to Roman rule and violently sought to eradicate it by
generally targeting Romans and Greeks. Zealots engaged in violence against
other Jews were called the Sicarii. They raided Jewish habitations and
killed Jews they considered apostate and collaborators, while also urging
Jews to fight Romans and other Jews for the cause. Josephus paints a very
bleak picture of their activities as they instituted what he characterized
as a murderous "reign of terror" prior to the Jewish Temple's destruction.
This says A LOT about what Jesus was able to accomplish in assembling his 12. Considering he had a tax collector, and a Zealot, who under other circumstances would target such a tax collector as a "collaborator". Consider the teachings of Jesus, and imagine how much of a life change that this Simon had undergone. Things like "Love your enemy", and "Give to Caesar what is Caesar's". It doesn't say that Simon was a Sicarii, but there were probably some shared core beliefs.
I am not a Bible scholar, but I think Zealot means more then just being zealous for Jesus. It is capitalized in the passage you shared.