Proteins are gotten from the food we eat. They are made up of building blocks called amino acids which are like chains of beads held together by peptide bonds. When we eat foods that contain proteins such as fruits, veggies, grains, nuts and seeds, digestive juices on our body breaks them down back into these building amino acids. They body now uses them to synthesize its own amino acids for proteins needed by the body.
Various amino acids have been discovered. Twenty two of them are important and the body can synthesize thirteen out of them while the rest are gotten from the protein food we eat. These proteins are found everywhere form our tissues, muscles, cells etc.
While some are of the opinion that animal protein like meat, milk, eggs, etc are the best sources of protein, other school of thought believe more in plant based sources of protein.