If you didn’t notice that there is not such a feature in your Gmail account like creating a customized folder, here is the statement: you really cannot add your own folders where some certain emails are collected.
What is so unusual about the Gmail interface and design? All the features on the places, where other mail applications have. But there's something tricky about Gmail anyway... You can star a message, you can mark a message as important… You have the Sent folder, you can even see the Spam folder in your Gmail account but NO customizable folders of your own!
Ok, I’m heating up the situation. The reality is that Google had invented the tags system long (just like free customizable websites) before user interface designers started using tag clouds on the web pages.
The explanation for folders refusal is simple, elegant and ingenious.
Let’s imagine the situation. Your Mom is going to celebrate her birthday and you are communicating with multiple relatives, friends, acquaintances, shoppers, restaurants, flower shops, photographers, etc. Potentially, you could have created a special folder and set up filters for each (!) conversation partner so that all messages were collected in one place. But it’s really tiresome, takes a lot of time and if there are indirect topics in some way connected with the birthday party that you should forward, discuss, answer…
In Gmail you don’t have to think about filters setup and rules creation. You create one label Mom’s Birthday preparation and tag all messages anyhow connected with the event with the label mentioned above. From now on, you can sort your conversations by this label and see all messages on the page. Cool, isn’t it?!
You can find more info on how to create labels in Gmail user guides. Moreover, you can create Gmail sub-labels to create a hierarchy which can be also very helpful.
While I was checking the information and read the user guides, I also found this table to which Google refers to prove that labels are better than folders:
Well, in my humble opinion, some of the reasons sound strained. But as a longstanding Gmail addict, I can say that it’s really not ergonomic and comfortable in sense of design and user interface to have a long list of folders that would make you scroll the inbox page for ages while you can apply dozens of labels to one and the same message and sort out the necessary email in seconds.
What I also aimed to say is that don't be afraid to change something in the design, introduce your own ideas and fresh thoughts in the interface just like Google does it!