Ways to share files:

• USB stick / external drive with lsblk and mount
• over the network
  • always encrypted: SFTP, FTPS, HTTPS
  • optionally encrypted: BitTorrent, SMB, WebDAV
  • unencrypted: FTP, HTTP, NFS, netcat
• with a version control system like Git or Mercurial
• with a synchronization program like rsync, Syncthing or Unison
• with a cloud (self-hosted or external internet service)
• with Bluetooth

See also:

• List of applications/Internet#File sharing
• Android#Transferring files
• Category:Synchronization