Can some one explain what the difference between

<META http-equiv="Content-Type" content="text/html; charset=UTF-8">

and

<META http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

?

Also, why do some sites omit these and use the following instead?

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">