Do you remember school blazers ? Not the barathea ones, the ordinary ones ? Well those are generally made from middling poor quality Melton.
Basically it's a fine tabby woven wool cloth that's been napped on the surface to create a smooth, hard wearing and generally showerproof surface.
Twill is a fabric that's woven like your denims. Instead of over under over under, the weft will go over two under one, over two under one, or maybe over three under one, over three under one, of the warp threads.
Sometimes the weaver will set the looms to create designs using that twill, diamond designs or chevrons, for instance.
More expensive to create, generally good quality fibres.
Tweed should be a specific term for a hand spun, naturally dyed, hand woven twill cloth made in Scotland or the English borders.
Nowadays it covers a multitude of fabrics.
Generally, natural dyed, or man made dyes in that range, colours.
Pure wool, twill woven cloth which has been at least partially fulled (washed and slightly felted so it bonds the yarns together a little).
It can be very coarse, rough and hard wearing, but it can also be the finest quality gents country suiting fabrics.
It's no accident that ghillies wear it
Does that help ?
cheers,
Toddy