By Will Wister after 1000s of hours studying psychology.
Short term effects:
Some effects may include a general alteration of conscious perception, euphoria, feelings of well-being, relaxation or stress reduction, increased appreciation of humor, music or the arts, joviality, metacognition and introspection, enhanced recollection (episodic memory), increased sensuality, increased awareness of sensation, increased libido, creative, abstract or philosophical thinking, disruption of linear memory and paranoia or anxiety. Anxiety is the most commonly reported side effect of smoking marijuana. Between 20 and 30 percent of recreational users experience intense anxiety and/or panic attacks after smoking cannabis.
Cannabis also produces many subjective and highly tangible effects, such as greater enjoyment of food taste and aroma, an enhanced enjoyment of music and comedy, and marked distortions in the perception of time and space (where experiencing a “rush” of ideas from the bank of long-term memory can create the subjective impression of long elapsed time, while a clock reveals that only a short time has passed). At higher doses, effects can include altered body image, auditory and/or visual illusions, pseudo-hallucinatory or (rarely, at very high doses) fully hallucinatory experiences, and ataxia from selective impairment of polysynaptic reflexes. In some cases, cannabis can lead to dissasociative states such as depersonalization and derealization; such effects are most often considered desirable, but have the potential to induce panic attack and paranoia in some unaccustomed users.
Some of the short-term physical effects of cannabis use include increased heart rate, dry mouth (cotton mouth), reddening of the eyes (congestion of the conjunctival blood vessels), a reduction in intra-ocular pressure, muscle relaxation and a sensation of cold or hot hands and feet.
Electroencephalography or EEG shows somewhat more persistent alpha waves of slightly lower frequency than usual. Cannabinoids produce a “marked depression of motor activity” via activation of neuronal cannabinoid receptors belonging to the CB1 subtype.
Acute effects while under the influence can include euphoria and anxiety.
When smoked, the short-term effects of cannabis manifest within seconds and are fully apparent within a few minutes, typically lasting for 2–3 hours. The total short-term duration of cannabis use when smoked is based on the potency and how much is smoked. Effects can typically last two to three hours.When taken orally, the psychoactive effects take longer to manifest and generally last longer, typically lasting for 4–10 hours after consumption.
More short term effects:
Aside from a subjective change in perception and, most notably, mood, the most common short-term physical and neurological effects include increased heart rate, lowered blood pressure, impairment of short-term and working memory, psychomotor coordination, and concentration. Long-term effects are less clear.
More short terms effects
Distorted perception (sights, sounds, time, touch)
Problems with memory and learning
Loss of coordination
Trouble with thinking and problem-solving
Increased heart rate, reduced blood pressure
Marijuana can be an intensifier
Some pot smokers report that their mood before lighting up a joint affects their mood while they are high. A person who is feeling happy may find that using marijuana intensifies those good feelings. Conversely, someone who is not in a good frame of mind who smokes weed may find that it acts more like a depressant.
Long term effects:
Some marijuana advocates acknowledge an increase in apathy or “amotivational syndrome” but they retort that the relationship has not been proven to be causal. In other words they suggest that apathetic people may be preferentially drawn towards pot.
There are concerns about marijuana and other mental health issues:
While cannabis has been correlated with the development of various mental disorders in multiple studies, these studies differ widely as to whether cannabis use is the cause of the mental problems, whether the mental problems encourage cannabis use, or whether both the cannabis use and the mental problems are the effects of some other cause.
It can cause cardiovascular problems 
chronic use is associated with some cardiovascular risk factors such as blood triglyceride levels and blood pressure
It may contribute to cancer
Studies show that someone who smokes five joints per day may be taking in as many cancer-causing chemicals as someone who smokes a full pack of cigarettes every day.
It may cause pulmonary problems
People who smoke marijuana regularly may develop many of the same breathing problems that tobacco smokers have, such as daily cough and phlegm production, more frequent chest colds, a heightened risk of lung infections, and a greater tendency toward obstructed airways. Cancer of the respiratory tract and lungs may also be promoted by marijuana smoke, since it contains irritants and carcinogens.
It may cause brain abnormalities (thx User)
The hippocampus, thought to regulate emotion and memory, and the amygdala, involved with fear and aggression, tended to be smaller in cannabis users than in controls (volume was reduced by an average of 12 percent in the hippocampus and 7.1 percent in the amygdala).
It can also cause various male fertility issues.