For people with disabilities, the easiest way to get back to work is often to be self-employed, which may allow the kind of flexibility in terms of hours and job duties that no other employer would offer. If you're receiving Social Security Disability Insurance benefits, though, there are rules that must be followed, both in terms of reporting work activity and in terms of how much you can earn before you risk being cut off.
For example, consider someone making crafts and selling them on the internet (through Etsy or eBay). Social Security looks at their earnings in that business as self-employment earnings. For self-employment, the most important number is the NESE (net earnings from self-employment, i.e., what the business earns after expenses), multiplied by .9235 (giving credit for the extra taxes self-employed people pay that a normal wage-earner's employer usually pays). There are some other work incentives under Social Security's rules that may wipe out more earnings, but that's a good number to start with.
$1,130 per month is what Social Security considers "gainful" employment (they adjust the number every year for inflation) - if you earn more than that in a month, you are considered to be "gainfully" employed, and not disabled for the purposes of Social Security disability. However, even earnings less than that can cause problems for those already on claim with SSA. Earnings of more than $810 in a month will trigger what is known as the "trial work period." Nine months of such earnings (not necessarily in a row) will complete your trial work period. After that, you enter the "extended period of eligibility," which is the 36 months (consecutive, this time) after your trial work period ends. During the extended period of eligibility (EPE), you can continue to receive your full disability benefit if you remain disabled, but Social Security will take away the payments you would receive for any months in which your calculated self-employment earnings (under the formula I described above) are more than $1,130 per month. At the end of the EPE, you risk being cut off altogether if you earn more than $1,130 per month in any single month.
This is all assuming that you are receiving Title II Social Security Disability Insurance (SSDI) and not Supplemental Security Income (SSI). Keep in mind that even if you follow all of the rules above, SSA sometimes screws this whole thing up, and may cause hassles for you down the road, despite their own rules.