ubuntu on an external drive

[MICHAEL WEAVER]

Can I use an external hard drive for an operating system such as Ubuntu 
or does the drive have to be an internal one for an operating system?
I ask because today I am hoping to buy a hard drive to I can duel boot 
Windows and Ubuntu so I can have my computer ready for the chap helping 
me install Linux when he next visits my flat.
My sister suggested I purchase an external hard drive but I am not sure 
if those ones are just for backing up or if they are used for desktop PCs.