Good explanation of gross and net debt here including an OECD chart of both for 2011. It’s a little out of date (March 2011) but essentially explains the current situation well.
This quote is the crux of the matter.
And here is where the assessment of government solvency gets more difficult: what you really want to do is not just to look at government debt but also at future revenues and liabilities.
As Katy said it’s not just the debt but how they can service it. The market sentiment is these countries in the Eurozone can only service their debts and maintain spending by borrowing more because their economies cannot perform and growth is expected to be almost zero. That is particularly true in the case of Spain, Greece, Ireland and Portugal.