can somebody please explain to me what is vocaloid, and is it a anime or manga?

A vocaloid is basically a voice synthesizer.Sadly they are not a anime,but there are fan made manga and there is on called Hatsune Mix by a artist named Kei.Vocaloid was a software that was created by the Yamaha Corp.
Vocaloid had Leon,Lola,Miriam,Meiko,and Kaito.The first 4 had good sales while Kaito didn't sell as good though.(Don't worry Kaito,we still love you.)
Vocaloid2 had Sweet Ann,Hatsune Miku,Kagamine Rin and Len,Prima,Gackupo,Megurine Luka,Gumi,Sonika,Miki,Kaai Yuki,Hiyama Kiyoteru,and Big Al.
There are also fan made vocaloids that are official.They are called UTAUS.Some examples are Kasane Teto,Akita Neru,and Akaito.
Well,I hope you get some idea of they are.To put it in a nutshell,they are singers.:)
Here's their first video posted on Nico Nico Douga.It's called Melt by Hatsune Miku.

A VOCALOID is neither anime, nor manga, but a singing synthesizer (or however you spell it) and is created by taking voice samples of phonetics and combining them in a program called VOCALOID and you create the lyrics and melody. The reason why you thought they may be anime or manga is because the most famous vocaloids are Japanese, so the illustrate them anime/manga style.