What's the difference between SCP and SFTP?

In a nutshell, SCP can only be used for transferring files, and it is non-interactive (i.e., everything has to be specified on the command line). SFTP is more elaborate, and allows interactive commands to do things like creating directories, deleting directories and files (all subject to system permissions, of course), etc.

From Wikipedia:

Compared to the earlier SCP protocol, which allows only file transfers, the SFTP protocol allows for a range of operations on remote files – it is more like a remote file system protocol. An SFTP client's extra capabilities compared to an SCP client include resuming interrupted transfers, directory listings, and remote file removal. [1] For these reasons it is relatively simple to implement a GUI SFTP client compared with a GUI SCP client.

and

Although both SCP and SFTP utilize the same SSH encryption during file transfer with the same general level of overhead, SCP is usually much faster than SFTP at transferring files, especially on high latency networks. This happens because SCP implements a more efficient transfer algorithm, one which does not require waiting for packet confirmations. This leads to faster speed but comes at the expense of not being able to interrupt a transfer, so unlike SFTP, SCP transfer cannot be canceled without terminating the session.

From a purely command line perspective :

• scp doesn't have an interactive mode nor can it read command scripts, that means everything must be written on the command line.
• sftp has an interactive mode and can read commands from a file.

Other important difference between the 2 commands is that sftp cannot put a local file to a remote location using a single command line, although it can get remote file, while scp can do both.

sftp get remote file

sftp user@host:/path/to/remote.file [/path/to/local.file]

scp get remote file

scp user@host:/path/to/remote.file [/path/to/local.file]

scp put remote file

scp /path/to/local.file user@host:[/path/to/remote.file]