When I was a kid the family dog was always coming home loaded with burrs in the fall and winter. Dogs ran loose in the neighborhoods back then and added their own part to moving the native plants around. I remember having to comb and brush and gently pull those burrs out of my dog's fur and the other day while out walking my latest generation of dog we encountered some burrs which got me thinking about them again.
Burrs are technically seed heads with little bristles or hooks. They are known as "hitchhikers" because they attach to fur, clothing, etc. and get carried to a new location where hopefully they will be dropped or rubbed off, starting a new planting in a new location. Many different plants have these burrs but the most common in our area are teasel, which is a taller plant and shown in the bottom picture (It is the one with the cone like seed heads. The other plant with the basket shaped seed head is Queen Anne's Lace) and these shown in the snowy photographs which are the seeds of American Burdock, a plant also known for its medicinal and edible qualities.
A fun fact about burrs is that velcro was developed by observing the way the hooks of burrs stuck to other materials.