There are three types of
data here: city-specific data, city characteristics data, and metadata.
The city-specific data consists of daily time-series of mortality,
pollution and meteorological data for three age-groups. Each city's
data is stored in a separate compressed comma delimited (UNIX) text
file. You can see descriptive statistics for a small number of
variables available for each city here and can download the data
below.
Along with the city-specific data you can also download city
characteristics data. This is a collection of tables giving
information about each city, such as, their latitude and longitude,
census demographics, how cities are defined in terms of couties
etc. Such data becomes increasingly important as you move from
analyzing one city to analyzing the whole collection of cities.
Finally you can download the metadata. This includes "data about the
data", such as a list of all the tables included in NMMAPS, a list of
all the variables, labels for variables, and labels for variable
values.
The documentation page gives
a better idea of what the data look like.