I wanted to install Google Goggles (to look at those evermore ubiquitous "square bar codes"), and so I went to my list of apps on my Android phone and looked for the Market app.
... and it wasn't there.
I checked again...
... and it still wasn't there. Furthermore, nothing looked like it could have been the Market app.
Okay, maybe someone's figured this thing out. Let me check online.
*check, check, check*
Huh. If you install Google Play, the Market app is removed. So, all I have to do is open Google Play?
*open Google Play*
Well, THAT'S simple. :-)
Furthermore, Google Play allows you to use the interface of your own web browser to assess what apps you have and install new ones.
IN SUM: If you installed the Google Play app, then you no longer have Market, but Google Play is what you use as Market (except that now you can control it through your web browser ... if you linked your phone to a Google account).