What Restriction Enzymes Are
Did you know that viruses attack bacteria, just like they attack us? Viruses can attach to the bacteria and inject them with their DNA. This is the same thing viruses do to humans, but the viruses that attack DNA are different. Viral attacks can damage or even destroy bacteria cells, so they developed a defense--restriction enzymes! Restriction enzymes are chemicals (proteins more specifically), that can recognize DNA and cut it in certain places. If the bacteria manages to chop up the viral DNA, the virus cannot do it harm and the bacteria remains safe!
Different Enzymes, Different Cuts
Different bacteria produce different restriction enzymes and each enzyme recognizes and cuts DNA at different places. For example, EcoRI comes from E.coli bacteria and cuts DNA wherever it finds a GAATTC. HindIII comes from a bacteria called Haemophilus influenzae and recognizes and cuts at AACGTT. There are thousands of restriction enzymes and each cuts at a unique site.
Using Restriction Enzymes on Human DNA
So why do we use restriction enzymes to cut human DNA? Every human's DNA is ~99.9% identical, but unless you are an identical twin there are small differences between your DNA and that of every other human on the planet. The differences are called SNPs (single nucleotide polymorphisms). When we cut DNA with restriction enzymes, it will sometimes cut the DNA of different people in different places, leaving different sized pieces. These are called RFLPs (restriction fragment length polymorphisms). We can then use Gel Electrophoresis to visualize the RFLPs and see those differences.
Using Restriction Enzymes on Bacterial DNA
Scientists also use restriction enzymes to genetically engineer (plasmid) DNA and therefore alter life forms. A plasmid can be cut open using an enzyme and then a gene of choice can be inserted in. The plasmid can then be inserted into cells and can affect the proteins they produce, and therefore their traits. See more about Genetic Engineering and also about Gene Therapy.