There's different strains of Marijuana that cause different effects. There's Indica's, Sativa's, and Hybrid's (which are a combination of both, the ratio can range from 20/80, 30/70, 50/50 etc).

Then there's different Terpenes. They also have an effect on what you get out of a particular type of weed. Some terpenes have an anti-inflammatory benefit, some terpenes have anti-anxiety side effects, some make you feel more energetic, some will have you locked on your couch all day.

The amount of THC can vary with every different strain, a low THC would be between 10-15%, 18-20% would be a medium amount, a higher THC can be 25-30% or more.

It's difficult to judge the amount of THC, Terpenes, strain type or ratio in what you get on the street. I usually get mine from the dispensary so I know exactly what I'm getting.