what is the difference between a source package and a package (want alsa-lib in stretch)

[Dan Hitt <dan.hitt@gmail.com>]

I'm trying to build some software, and one of the long list of items
to install prior to configuring is 'alsa-lib'.

I tried
    'sudo apt-get install alsa-lib'
but got the message
    'E: Unable to locate package alsa-lib'

So i looked around on the internet, and saw a page for an alsa-lib
package on debian stretch:
https://packages.debian.org/source/stretch/alsa-lib

I had just installed libjansson-dev, so i didn't think i had anything
wrong with my local state, and it had a similar page
https://packages.debian.org/sid/libjansson-dev

However, the page for jansson identified it as a 'package', while the
page for alsa-lib identified it as a 'source package'.

So i suppose that a source package needs to be installed in some other
way, different than the way a 'package' is installed?

It's conceivable that i already somehow have alsa-lib installed in
some form (e.g., 'dpkg --get-selections' shows i have an alsa-utils).

Anyhow, would appreciate any clues.

I imagine that what i need are the libraries and the include files in
order to build this other software that asks for alsa-lib.

Thanks for any info!

dan