CBD is gaining a lot of attention of late for its numerous medicinal properties and while not as potent as THC in some areas it does appear to be more effective in other areas. Even after decarboxylation CBD has no recognised psychoactive effects, making it more desirable

than THC for many patients who wish to avoid the high.
CBD does pass the blood brain barrier but unlike THC does not activate the CB1 receptor (which is responsible for the psychoactive effects of THC). Though this inability does also prevent it from utilising some of the therapeutic benefits associated with CB1 receptors it has been demonstrated to utilise many other pathways for significant medicinal benefit.
CBD, like all neutral cannabinoids, is not produced directly by the plant but rather is a result of decarboxylation of CBDA.

CBD has a large number of recognised medicinal properties. It is one of the main focuses of the US patent 6630507, where it is described as a powerful anti oxidant and neuroprotectant. CBD has demonstrated significant anti cancer effects for various types of cancer, though not as effective as THC for many cancers, like certain forms of brain cancer, CBD does appear to improve the efficacy of THC on most cancer types and has demonstrated superior efficacy for certain types of cancer, specifically breast cancer.
In addition to the significant anti cancer effects, CBD is recognised as an anti inflammatory, anti bacterial, anti depressant, anti diabetic, anti nausea, anti epileptic, antispasmodic, anti ischemic (reduces risk of artery blockage), Anxiolitic (reduces anxiety) and anti psychotic. It also has been shown to be a potent neuroprotectant and Antipsioratic (can treat psoriasis) and can stimulate bone growth.
CBD moderates the psychoactive effects of THC, but does not appear to hinder the medicinal properties of THC with many studies confirming that it actually enhances many of the therapeutic actions of THC.

CBD is among the most established of the anti cancer cannabinoids, currently only rivalled (and in some cases surpassed) by THC. Most of the studies on CBD have been done in the last 10 years with both stand alone anti cancer effects and synergistic effects that enhance the anti cancer properties of THC demonstrated.

Though the evidence is primarily anecdotal at this stage, the real life successes treating various forms of epilepsy have prompted massive interest in the use of high CBD extracts for treating various forms of epilepsy, with the US regulatory agency, the FDA, actually approving trials for a high CBD extract produced by GW Pharmaceuticals as a treatment for epilepsy (though unpatented cannabis extracts have not been approved for use by the FDA)

CBD has demonstrated numerous anti diabetic effects, as well as other therapeutic properties beneficial for sufferers of diabetes.

Like THC, CBD has shown the potential to aid bone repair and growth with an obvious entourage effect with both cannabinoids working together to enhance efficacy.

CBD is also the subject of great interest for treating various bowl related problems and is seen as a potential treatment for Crohn's and IBS.

Though other cannabinoids have shown potential benefit for various forms of mental illness, CBD is currently the subject of the most interest as an anti psychotic and is currently being investigated as a potential treatment for schizophrenia.

As is the case with most, if not all, of the other cannabinoids, cannabinoid acids and even terpenes, the benefits of CBD are often massively enhanced by the combination of other cannabinoids. THC has demonstrated the most notable synergy with CBD, often drastically enhancing efficacy for many conditions. THCA has also demonstrated tremendous synergy with CBD, as have some of the terpenes (with Myrcene and Linalool appearing to be the most significant in this area).
The anti cancer effects of CBD are drastically improved by the presence of THC for many types of cancer with CBD virtually ineffective on its own for some types of cancer yet showing significant benefit when applied in combination with THC.
Even the increasing evidence supporting its use as an anti epileptic medicine seems to suggest that the presence of THC, and other cannabinoids, is vital to maximise the effectiveness.