Files are encrypted/decrypted on your browser which enable only you open your files. Even we cannot open them since all keys are also encrypted with your own secret key.
Your files are stored on InterPlanetary File System - IPFS, which is open-source, peer-to-peer decentralized system for storing and accessing data.
dfile.io does not require any personal information other than username and password. You can even share your files anonymously.
While many storage services require full storage cost of files to each user even if the files are shared, dfile allocates prorated storage cost based on how many users share. You can store huge tera-byte files with affordable fee.