Q:

How to make only limited letters can be typed into  with jQuery?

For example,to allow only digital numbers to be typed in:
<input type="text" id="target" />

A:

Use the keypress() event. This example prevents characters that aren't digits (0 = 48, 9 = 57).
$(function() {
  $("#target").keypress(function(evt) {
    if (evt.which < 48 || evt.which > 57) {
      return false;
    }
  });
});

See this list of Javascript key codes.

