Tagliatelle

Freshly extruded egg pasta cut in narrow ribbons wider than fettuccine. These long ribbons of pasta are very similar to fettuccine.  They go well with a hearty meat sauce. 

Substitutes: fettuccine (slightly narrower, almost identical) OR pappardelle (a wider version)

Fettuccine

Freshly extruded egg pasta in long flat strips wider than linguine. These are long, relatively thick ribbons of pasta.
Egg fettuccine goes well with most sauces although cream sauces seem to be a popular marriage.
Substitutes: tagliatelle (narrower and thinner version), linguine or spaghetti.