What is Instagram, what is its benefit and how to make best use of it ?

Guys,
I see few of my friends are very active on instagram, I find them regularly talking about it. But I have never used that service, from search I can see that its about sharing photos through mobile but if I open their site then there is option to login but there is no option to create account ? So, it can't be used on PC ? and only on mobile ?

And say , if I creates instagram account, what will be its benefits . Also want to know what is its best use, why people use it.. like people use FB for friends , companies use FB for promoting their brands.. what is instagram's best use ?