NIST defines cloud computing through these five essential characteristics:
In addition to these characteristics, the Cloud Security Alliance also highlights multi-tenancy as an important element for cloud computing.
Cloud computing architecture can be visualized through models such as:
References:
http://csrc.nist.gov/publications/nistpubs/800-145/SP800-145.pdf
https://cloudsecurityalliance.org/guidance/csaguide.v3.0.pdf