Tax shelters reduce current tax liability by offsetting income from one source with losses from another source. The IRS allows some tax shelters, but will not allow a shelter which is "abusive."
An abusive shelter generally offers inflated tax savings which are disproportionately greater than your actual investment placed at risk. Generally, you invest money to generate income. However, an abusive tax shelter generates little or no income, and exists solely to reduce taxes unreasonably for tax avoidance or evasion.
In comparison, a legitimate tax shelter often produces income and involves a risk of loss proportionate to the expected tax benefit. Abusive tax shelters are often marketed in terms of how much you can write off in relation to how much you invest.
This "write off" ratio is often much greater than two-to-one as of the close of any of the first five year ending after the date on which the investment is offered for sale. A series of tax laws have been designed to halt abusive tax shelters. An organizer of a potentially abusing tax shelter who doesn't maintain a list of investors is subject to penalty of $50 per failure, per person, unless due to a reasonable cause and not willful neglect.
Any person participating in the sale or organization of an abusive tax shelter may be penalized up to the lesser of $1,000 or 100% of the Gross Income derived or to be derived from the activity.
Courtesy of the Internal Revenue Service. http://www.irs.gov/faqs/faq-kw195.html