The hot weather that occurs in summer is caused by the Earth's axial tilt, which affects how much sunlight different parts of the Earth experience throughout the year. During summer in the northern hemisphere, that half of the globe is tilted 23.5 degrees toward the sun, increasing both the length of the day and the amount of time the sun has to warm the surface.
One important feature of the summer months in the northern hemisphere is hurricane season. The warmth the sun provides for the ocean causes an increase in the amount of moisture and heat that rises into the atmosphere, triggering cyclonic storms. Cyclone season runs from June 1st to November 30th, and some of the most devastating storms in history have occurred during the hottest months.
The longest day of the year in the northern hemisphere occurs on June 21st, and while this would seem to indicate the center of summer, meteorological summer tends to lag due to the heat-absorbing properties of the oceans. As the amount of sunlight hitting the northern hemisphere increases, the oceans absorb much of the extra heat, releasing it later into the summer. This is why the hottest months are often June, July and August, despite peak heating from the sun occurring in late June.