There are many definitions of a "digital library." Terms such as
"electronic library" and "virtual library" are often used synonymously. 
The elements that have been identified as common to these definitions[1] are:
-  The digital library is not a single entity;
-  The digital library requires technology to link the resources of many; 
-  The linkages between the many digital libraries and information 
services are transparent to the end users;
-  Universal access to digital libraries and information services is a 
goal;
- Digital library collections are not limited to document surrogates:
they extend to digital artifacts that cannot be represented or 
distributed in printed formats. 
Purposes
The purposes of a North American digital library system[2] are:
-  to expedite the systematic development of: the means to collect,  
store, and organize information and knowledge in digital form; and of 
digital library collections in North America;
-  to promote the economical and efficient delivery of information to 
all sectors of North American society;
-  to encourage co-operative efforts which leverage the considerable 
investment in North American research resources, computing and 
communications network;
-  to strengthen communication and collaboration between and among the 
research, business, government, and educational communities;
-  to take an international leadership role in the generation and 
dissemination of knowledge in areas of strategic importance to North America;
- to contribute to the lifelong learning opportunities of all North Americans.
[1] See: Drabenstott, Karen M.  Analytical review of the 
library of the future, Washington, DC: Council Library Resources, 1994. 
[2] Adapted from The CAN-LINKED Initiative, a proposal 
for the co-ordinated development of a distributed national digital library
system in Canada, prepared by a group of academic and research
libraries.  February, 1995.