Proteins are a chain of amino acids. Proteins can have up to several hundred amino acids. Snippets of proteins (peptides), 7-15 amino acids in length are important in immunology. There are 20 types of amino acids. Each is assigned a letter (1 letter code).
Antibodies are proteins that can bind to peptides that have a specific amino acid sequence. Such a target peptide is known as an epitope. When an antibody binds to a peptide (which is part of a protein, which in turn may be part of a cell surface), it can trigger an immune attack on the cell. If the cell were a bacterium, the bacterium would be killed.
Humans (like all organisms) are made of numerous proteins (self-proteins). So, we have self-proteins, self-peptides and self-epitopes. In a healthy person, the body will not make antibodies that bind strongly to self-peptides (self-tolerance).