A short term medical policy is a plan to cover you for the "short term" or to bridge a gap until your major medical policy comes into play. A short term plan typically lasts anywhere from 10 days to 6 months, depending on the carrier and plan. It is not a major medical plan and thus does not always follow the new ACA guidelines.
You can be turned down for certain pre-existing conditions or a pregnancy, which would not be the case for a major medical plan. Short term plans are going to be cheaper than a major medical plan, but it's sole purpose is that you at least have coverage in a gap or period where a major medical plan would not cover you, such as going over seas for a period of time or being in between jobs before employer benefits kick in.