In the housing world usually most homes have mortgages. A Mortgage or A Deed of Trust with a Note is a lien against that property. However legitimate mortgages are not a problem. They technically are clouds, but they are so common that it's just expected for them to exist. And they are easy to remove; they just need to be paid off before property ownership transfer.
Actually in many states any other lien except for government liens will be wiped out when buying Tax Liens and later foreclosing on them or when buying Tax Deeds. Make sure you check with the county officials and the statutes for your state. But if your state is one of them, then you can easily go and pretty much disregard mortgages. They just mean that the property is more likely to be redeemed because the entity holding the mortgage will not let the property go for taxes and rather redeem it themselves. But in that case you get your money back PLUS a great interest rate (usually).
When buying properties directly from the owner, you will need to find out the pay-off amount so you can determine if this property is worth your efforts. This is easily obtained by having the seller sign a simple form authorizing you or the title company to talk to the lender to get the figure, and then basically as part of the closing process you pay, or the title company pays, it off (and of course that amount is subtracted from what the seller gets for the property).