They're both text files of thing=thing\n which apply to an MLT thing, which change how the 'mlt service' works.
And how can I add a new profile? And a new preset?
A profile helps set the required properties of the end output. A preset is just a collection of options which can apply to any mlt service.
You can create a new preset by putting the key=value pairs into a file anywhere, then using the absolute path of the file, where you would normally specify a known-preset-name.