Life Cycle Assessment (LCA) is the science of measuring the environmental impact of products and services over their entire life cycle from cradle to grave. A Life Cycle Assessment model is actually at least three different models.

1. A technosphere input-output models that describes the human activities in extracting, modifying, discarding and using energy and materials
2. An emissions and resource model that estimates resource degradation and pollution emissions linked to the technosphere model, for example the emissions of air pollutants from burning a certain amount of fossil fuel in a boiler
3. An environmental impact model that uses the resource and emission model outputs to estimate the impacts on the environment and the economy. The classic example is that of global climate change which is modeled in accordance with the work of the Intergovernmental Panel on Climate Change, yielding units of CO2 equivalents. 

All of these outputs of these models are expressed relevant to the functional unit of the product in question, for example covering a square meter of floor for 30 years, or delivering 1 kWh of energy to the point of use. The use of a functional unit is what allows LCAs to provide the environmental results to the user in terms that are relevant to the user.

A carbon footprint is a kind of LCA that only reports Climate Change iimpacts. Normally LCA covers all relevant impact categories, and it is therfore known as the holistic yardstick of environmental performance. Life Cycle impact assessment results are indicators of impacts, not measurements of actual impacts on the environment. For example, LCA does not measure the loss of particular ecosystems. Rather the LCA provides indicators of aggregate losses all ecosystems across the life cycle.

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