MDxx refers to a class of substituted methylenedioxyphenethylamines. MDMA (ecstasy) is the prototypical compound of this series - as such, drugs of the MDxx series are considered analogues of MDMA. Not all of these compounds are explicitly listed as controlled substances. Some have been, and continue to be, available as recreational research compounds for casual use.
Popular drugs of this series include MDMA, MDA, MDPEA, MDPV, MDAI, MDEA, and bk-MDMA; a few of which are now controlled substances in the US, and a few which may not yet be.
MDxx compounds are generally taken as recreational or party drugs. They produce a range of sympathomimetic (i.e. stimulant), euphorigenic, empathogenic, and psychedelic effects similar to ecstasy and amphetamine.
Considering MDMA as a prototype, MDxx compounds produce their desired effects by increasing extracellular concentrations of the monoamine neurotransmitters in the brain (dopamine, norepinephrine, and 5HT), this is being done by various means, including reuptake inhibition and transport phosphorylation.
The empathogen properties set many of these compounds apart from psychedelics such as LSD or DMT, and from traditional stimulants like amphetamine; these effects, marked by increased empathy, sociability, and affection, have been linked with the reversed out-pouring of 5HT into synaptic terminals (i.e. 5HT phosphorylation) - MDMA is a relatively potent serotonin releasing agent - as opposed to hallucinogens such as LSD which only bind and activate serotonergic receptors, or as opposed to stimulants like amphetamine, which don't exhibit nearly this degree of serotonergic action. MDMA also produces weak activation of the 5HT receptor, and its serotonergic properities are believed to induce increased active levels of the hormone & neuromodulator oxytocin.