Network subnetmask

Hey!

I'm currently self-learning Cisco, but the book is pretty fuzzy about "subnetmasks".
See, if I am to for example create a subnetmask that allows me 14 networks for a Class-C network, do I write a subnetmask like 255.255.255.0, or am I allowed to write it like 255.255.240.240 (also is this a subnetmask that allows me 14 networks?).

Many thanks!

I mean, does the subnet mask 255.255.255.240 /28 provide me with 14 usable sub-networks? If not, how does it look?