Determining User Browser(NS Or IE) ?

I want to make an if stament that will do a thing if the browser is Internet Explorer. Else if the browser is Netscape will execute other code. How can I make that?

if (Internet Explorer)
{execute this code}

else if (netscape)
{execute this other code}

?

PS-also, is there a tutorial or reference that sais what commands are supported by each kind of browser?(most important for me at the moment is knowing wich commands are supported by browsers to show and hide layers..)