Re. Wordpress wp_options | active_plugins, what does the 's' refer to in:

[Muttly]

i:10;s:20:"download-counter.php";? Hi,

I guess the subject line says it all. The 'i' is clearly sequential, but I'd like to know what the 's' refers to and how it's determined.

This is obviously a technical question so kudos to anyone who knows!