Street art is made in a variety of ways. We'll go over the three ways of doing it (graffiti has its own definition.)
One way of producing street art is stenciling, a stencil is made by cutting out a printed or hand drawn picture in specific areas as to produce an image after applying spraypaint. Street artists like Banksy use this technique a lot.
The second way of producing street art is called wheatpasting, which is when you use a homemade paste to roll over an image you printed (the paste only dries in thin coats, there are many tutorials on how to make it on YouTube.) Shepard Fairey is a legend of wheatpasting.
The third way of producing street art is called sticker art, also called slap tagging or just simply slapping. Stickers are usually made with USPS Priority Mail stickers or using homemade stickers with homemade adhesive. This is very similar to wheatpasting but it does not use wheatpaste. Stickering is best done in broad daylight since it's quick and easy to do and people don't give a shit about stickering but flip the fuck out when you start spraypainting a wall. A famous sticker artist is Space Invader.
While some of it is made illegally and is associated with vandalism, a large amount of the murals are commissioned and legal.
The association with vandalism and graffiti leads some people to assume that these murals are rebellious.
Often made with aerosol cans or stencils. The imagery and style is often conventional illustration.