What it is
From the website:

The City's photo archive contains over 2 million photo records that date from the late 1800's. This web site contains a growing collection of those photos. All archive photos may be searched by keyword and date. Archive photos which have been assigned a geographic location are also searchable by proximity to an Address, Intersection, Place Name, or Neighborhood.

Pros
The search is amazing - search by address, landmark, keyword, etc.

Cons
Like any site of this kind, the controlled vocabulary for searches isn't perfect, but very close.

Philly Connection
See photos of Philadelphia.

What you can learn from it
Find out what your neighborhood looked like throughout the years.