So, I'm Italian and I'm gonna say a first thing:
they are NOT called macaroni. The name is Maccheroni.
- Maccheroni is a type of pasta made like this:
http://www.ilcucinario.it/RICETTE%20STAM…
The are empty inside and they have lines on the surface that go from one end to the other.
- Then Spaghetti are the classic italian pasta:
http://www.isoladischia.net/public/artic…
Just long lines (about 30cms). As easy as that.
- Noodles are just like spaghetti but they are chinese and are made from rice, while all kind of pasta is made from wheat. They have the same shape but are a little bit thinner and they are white (as made from rice).
The one you find in shops are not proper noodles, now with noodles they all mean "spaghetti" but in fact it's not like that.
- Pasta is the kind of food (spaghetti, maccheroni, fusilli, farfalle etc.etc.) . There are many types of pasta: the usual one is made from wheat and water and other little stuff; another popular one is the one made also with eggs in the dough (really good) and then you can find a lot of other types.
Here are some kinds of popular pasta outside Italy:
http://www.isoladischia.net/public/artic…
http://images.google.it/images?hl=it&um=…
http://www.buttalapasta.it/wp-galleryo/f…
http://images.google.it/images?hl=it&sou…
NOTE:
every kind of pasta is named with the plural form of the name, so Spaghetti is the kind but if you wanna say just one single line you say Spaghetto. Same for maccheroni, farfalle, fusilli and all the others (maccherone, farfalla, fusillo etc.etc.)