Greece has a bad economy and it threatens to bring down the entire economy of Europe. If richer countries like France and Germany have to bail out Greece, it will cost their citizens. Greece should have to pay its own debts and should not rely on its more responsible neighbors for help.
Yes, the euro is at risk. The whole world economy is at risk right now, thanks in large point to massive debt caused by welfare-state socialist economies that western Europe employs that they can't pay for. The United States is flirting with this same style of socialism, but seem to be waking up thanks to the disastrous roll out of the Affordable Care Act (ObamaCare).