Forbearance is available for people who don't qualify for a deferment, or who have used up the maximum amount of time that the lender will allow for deferments. Because you continue to accrue interest during periods of forbearance, forbearance is generally easier to qualify for then deferment.
According to the Department of Education, borrowers can qualify for a forbearance if they are willing but unable to make their payments. This could be for any number of reasons: your income might be too small but not small enough to qualify for deferment, or you may have too much debt and be unable to pay it all. Sallie Mae has similar qualifications for deferments: you must want to pay, but not be able to pay.
Some lenders, including Sallie Mae, may charge a fee for putting your loans into forbearance. Other lenders require you to make at least one "good faith" payment when putting your loans into forbearance. Forbearance generally lasts for a one year period of time, and you may then need to apply for forbearance again and either pay another fee or make another "good faith" payment.
Interest accrued during periods of forbearance becomes "capitalized" or added on to the principle, so putting your loans into forbearance means you will end up paying back more money in the long run. You have the option of paying interest during the period of forbearance to prevent this from happening.
Both Sallie Mae and the Department of Education, as well as many other lenders, put a limit on the number of years your loan can be in forbearance before you must begin making payments.