What is it and which one should I choose when making webpage for people from Estonia and english-speaking countries?

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

charset states what set of characters should be used to display the code within the document. the command goes between the HEAD tags.

different values for charset can be found here...
http://people.netscape.com/ftang/meta.html

dunno which one is right for your situation though. good luck!