As described in the central dogma, DNA encodes the instructions to assemble amino acids into proteins. Those instructions make up a universal code of sorts for all livings things (with some minor modifications in a few cases). To appreciate just how amazing this code is, understand that there are 1.5 × 10^84 possible codes. That exceeds the number of atoms in the universe. Yet somehow life settled on a single code - which turns out to be one of the most efficient.
Here is how the code works: The DNA is read in groups of 3 'letters' called codons. The messages begin with a 'start' codon that usually encodes the amino acid methionine. The messages end with a stop codon that doesn't code for any amino acids. The protein building machinery uses special RNAs called transfer RNAs (tRNAs) that have the complementary match to the RNA and link to the specified amino acid. This system is truly remarkable.
Amino acids are the building blocks for proteins. There are 20 or so that are used by all living things. Some of the more simple ones are naturally occurring and have even been found in meteors from space. Others are far more complex. Small strings of amino acids linked together are called peptides. Longer strings of amino acids are called proteins. Some proteins, often called enzymes, can carry out chemical reactions. Others work like building blocks to make larger structures or function as receptors to detect signals. Proteins are capable of so many different activities that it's hard to even fathom. Kind of like a biochemical Lego set.