Tax avoidance is legal. Using an IRA would be an example of tax avoidance; a government-approved way to defer taxes on retirement savings.
Tax evasion is illegal.
However, this piece on the UK's HSBC bank helping clients stash cash in Swiss accounts seems to be unclear what is going on here. If it is mere avoidance, then the bank should be in the clear.
Unlike the US, who taxes Americans on money made wherever they earn it, most foreign countries only tax income earned in their country. Thus, money held in foreign investments would be taxes by the country the investment was in rather than the investor's home country.
That's why a number of high-income Europeans set up shop in income-tax free locales. Monaco is one example of a flag of convenience for folks, giving reasons beyond the casinos and coastal living for being there; they have sales tax (value-added taxes to be technical) and a steep FICA-like payroll tax, but no income tax.
Living there is tax avoidance for many folks but not tax evasion.
The BBC piece seems to blend the two terms. It would be helpful to clarify what if any of the HSBC misdeeds are abetting evasion, if any.