A Tag Library Discriptor(TLD) is an XML like file (like web.xml) used to describe the functions(EL) and tags provided by a user defined tag library.
The tag is the actual element that performs some customizable functionality through its attributes (like <
jsp:setproperty>, is used to set property of a
java bean) in a JSP file.
I hope that this explains the basic difference though the topic has many other things to be learnt.