The effects of Marijuana are manifold.
The immediate effects that most users seek is the effect of getting high. The world suddenly can be seen from a slightly differrent angle. Getting high can be have many positive effects or can be a nightmare.
Humans seem to have a need for changing their inner state. Psychedelic drugs have been used since thousands of years to alter peoples state of mind.
7000 – 9000 year old rock paintings that show the usage of hallucinogenic mushrooms have been found in the Sahara desert. In that case, the use of the drug has been one of the inspirations for that rock art.
Marijuana may currently have a bad reputation in most societies of this world. In countries like Mauritius or Bali you are likely to go to prison for life if you get caught with it.
Australia lets you have 2 plants in your garden without necessarily sending you to court. The judges have since years been flooded with thousands of small Marijuana cases so they loosened the law. (not anymore, law was changed back to no plants allowed)
The Netherlands allow you to buy quality Marijuana in a coffee shop and enjoy the positive Marijuana Effects right there.
A special and interesting area is the connection of Marijuana and Sex.
To everything in life, there are side effects to Marijuana. The unwanted side effects are described here.