This from Cisco's Internetwork Design Guide

Using the Hierarchical Design Model
A hierarchical network design includes the following three layers:
    * The backbone (core) layer that provides optimal transport between sites
    * The distribution layer that provides policy-based connectivity
    * The local-access layer that provides workgroup/user access to the network

Function of the Core Layer

The core layer is a high-speed switching backbone and should be
designed to switch packets as fast as possible. This layer of the
network should not perform any packet manipulation, such as access
lists and filtering, that would slow down the switching of packets.





Function of the Distribution Layer

The distribution layer of the network is the demarcation point between
the access and core layers and helps to define and differentiate the
core. The purpose of this layer is to provide boundary definition and
is the place at which packet manipulation can take place. In the campus
environment, the distribution layer can include several functions,
such as the following:

    * Address or area aggregation
    * Departmental or workgroup access
    * Broadcast/multicast domain definition
    * Virtual LAN (VLAN) routing
    * Any media transitions that need to occur
    * Security

In the non-campus environment, the distribution layer can be a
redistribution point between routing domains or the demarcation between
static and dynamic routing protocols. It can also be the point at which
remote sites access the corporate network. The distribution layer can
be summarized as the layer that provides policy-based connectivity.




Function of the Access Layer

The access layer is the point at which local end users are allowed
into the network. This layer may also use access lists or filters to
further optimize the needs of a particular set of users. In the campus
environment, access-layer functions can include the following:

    * Shared bandwidth
    * Switched bandwidth
    * MAC layer filtering
    * Microsegmentation

In the non-campus environment, the access layer can give remote sites
access to the corporate network via some wide-area technology, such as
Frame Relay, ISDN, or leased lines.

It is sometimes mistakenly thought that the three layers (core,
distribution, and access) must exist in clear and distinct physical
entities, but this does not have to be the case. The layers are defined
to aid successful network design and to represent functionality that
must exist in a network. The instantiation of each layer can be in
distinct routers or switches, can be represented by a physical media,
can be combined in a single device, or can be omitted altogether. The
way the layers are implemented depends on the needs of the network
being designed. Note, however, that for a network to function optimally,
hierarchy must be maintained.