masonry supply store

[Europhyllia]

ok silly question but what is the difference between mortar and cement?
I am wanting to do this egg crate/ rock cover for overflow boxes:
http://www.marcorocks.com/index.asp?...TS&Category=49
Having trouble finding exactly the mortar he's using there and am worried about using 'the wrong kind' so I was thinking maybe I could make cement work instead?
We've always used cement in more of a slushy form and mortar more of a paste so I would have thought mortar would be better for rocks, etc. but I am sure with less water cement would be kind of mortary...