Cannabis affects individuals in different ways.
Although THC is the primary active component, cannabis has over 250 psychoactive compounds according to researchers in Israel (see the journal Scientific American). Crops vary greatly in strength depending on their growing conditions, the genetic strain, the gender and stage of maturity of the plant, and how old it is since harvest.
Smoking, drawing it slowly and deeply into the lungs and holding your breath for a few moments, will bring on the effect within a few minutes. If you're non-smoker (cigs), you will need to suck in cool air at the same time, otherwise it will give you a burning feeling in the lungs. If it's mild you may need several deep inhalations.
If you eat it in cookies or drink it as a tea, the effects come on slowly, starting after half an hour, slowly increasing, and taking much longer to fade away - much better for keeping lungs healthy.
One of the effects is feeling sense perceptions, like flavours, more strongly. Most people get "the munchies" which is why it's excellent for appetite in people enduring chemotherapy for cancer. Some people feel a mild high, others peace and relaxation. For many, the increased sensuality enhances sex. Some feel energised and enthusiastic, others get sleepy. People with a predisposition to anxiety or schizophrenia may become paranoid. It reduces the symptoms of glaucoma and can help reduce many types of pain.
It definitely slows reflexes, so never use it if you intend to drive or use machinery.
Remember that the traces remain detectable in the blood for up to eight days, long after the effects have worn off. If you're in a state where it's not legal you won't want to have it in you if police are doing random driver tests.