How Cannabis works
Until 1993 we had no idea that this system in our bodies even existed, and are to this day are still studying the effects of chemicals on this network of receptors.
When you ingest cannabis it either goes to your liver, or into your blood stream. (This is the difference between the effects in smoking/eating/vapor, and why each "high" feels different.) once there it travels through your body, falling into "keyholes" in receptors located in the image above. CB1 receptors enable THC (tetrahydrocannabinol - the psychoactive compound in cannabis) to modulate the way your hormones/nervous system work. CB2 receptors are located throughout your entire body and are directly tied to your immune system, modulating the way those soft tissues function.
There are 483 organic chemical compounds found in the cannabis plant and each one does something different. Each "strain" (breed) of cannabis has a different level of each chemical, which in turn creates a different effect. Some strains are great anti-anxiety medicine, some will make you horribly paranoid. Some will cause you to eat everything in your house ("Munchies") which is great for cancer patients, and patients suffering from HIV/Aids. And again some strains (Predominantly African Landrace strains) work like an appetite suppressant. (THCv-high strains will do that.)
This is how using cannabis alongside chemo and radiation has been shown to be very successful for so many patients. Also how people with PTSD, epilepsy, Lupus, ALS, Alzheimer's disease, MS, and so many other diseases are feeling relief. But before we get into the disease/medicine-topic lets look at the cannabis plant as a whole.

This is your brain on Pot
"Smoking kills Brain cells"
"Pot will make you lose all ambition"
"Weed will make you a dope"
"Cannabis helps me study"

One of the above statements is true, but it likely won't be the one you think it is. (Disclaimer: Your experiences may vary - with studying. ;D) The other statements are not only untrue, but totally debunked. A lot of misinformation is -still- floating around about cannabis.

That is why I decided to spend my day off typing this all up. I'm not gonna get on my soapbox and start preaching from the rooftops because likely most of you don't give half of a shit about any of this. But if you -ARE- curious, and if you -DO- want current information, I added some links that will be very eye-opening.

Cannabis plant types
I did warn that there are is a lot to learn about cannabis (Kudos to you for still reading), lets dive right into the vegetation, here - so to speak.

There are 3 notable types of Cannabis plant;

• Cannabis sativa L.
• Cannabis indica Lam.
• Cannabis ruderalis Janisch

Foreword: A good bud tender will have tried everything on their shelf personally- and should be able to make good recommendations for your specific condition.

Keep a journal so you can keep track of the strains that work for you. and keep in mind that if this was a psychiatrist's office it could take up to 6 months to find a medicine that will help your specific ailment. Be patient, document everything.

Cannabis sativa L., also known as simply "Sativa"
Notable strains include: Super Silver Haze, Durban Poison, Maui Waui, Colombian Gold, Panama Red, Sour Diesel, Jack Herrer

Most often sought for its uplifting and euphoric properties, sativas often are best for mood disorders such as depression, bipolar disorder, social anxiety (strain dependent), and fatigue.

Frequently reported side effects: Paranoia, dry-mouth, red eyes, racing thoughts, hunger, sweating, palpitations (usually from anxiety)

Cannabis indica Lam. also simply known as "Indica"
(Pro tip: Indica: In-da-couch, easy way to remember its effects)
Notable strains include: Granddaddy Purple, Bubba Kush, Northern Lights, Blueberry (I), Master Kush (Aphrodisiac for some men), Purple Erkle, Death Star, Cannatonic #4
Most often sought for its relaxing, and pain-relieving properties. Indicas are also great for insomnia, anxiety, muscle spasms, nausea, and inflammation.
Common side effects: Tiredness, "couch-lock" (Myrcene), heavy eyelids (Dennis Leary style), Dry, red eyes, hunger, Thirst,
(Indicas are the best "first attempt" at trying cannabis because they are not prone to causing anxiety. Usually people who have tried cannabis and disliked it intensely started with a Sativa, and are understandably jaded from the experience. The sad fact is that in illegal states you often simply do not know what you are getting or what is in it/how it will effect you.)

Cannabis ruderalis Janisch Also known as "Ruderalis" is a lesser-known breed of cannabis, it is a cross between the Cannabis Hemp plant (that has no psychoactive properties, and is solely used to make consumer goods) and a cannabis sativa or indica plant. It is bred for its fast grow time and unique look.

Strains of cannabis that are intentionally bred for an express purpose that do not fall into the Sativa or Indica categories are known as Hybrids. Just like in middle school when your teacher showed you this chart: - that is essentially how people make strains for specific ailments, like Gorilla Glue #4 that was bred for menstrual cramps, or Charlotte's Web that was bred to help a little girl deal with her epilepsy.
( 43 min special on CNN:

)

Hybrids are a bit of a genetic crap chute. This is where you really rely on your bud tender to know their shit. Regardless of what the strain says it does, the conditions it was grown in, what chemicals were added to it, and how early it was picked/cured determine how the cannabis will effect you. That is why it is so important to find knowledgeable bud tenders. You'll go into the dispensary and end up worse-off sometimes. Call the shop first and ask some hard questions;

• Does your dispensary test each product each batch?
  If so where can I see these results?
• Are your staff members familiar with _______ <disease name
• Will there be time to give me a private consultation, as I am a first-time user and I will be asking a lot of questions
  (Sometimes I get new patients in the middle of a huge rush, And I will take my time explaining everything, which sometimes upsets the other patients who have to wait. Please Please call ahead.)

Sources:
https://www.leafly.com/explore/category-indica
https://www.leafly.com/explore/category-sativa
https://www.leafly.com/explore/category-hybrid