We describe the European Large Area ISO Survey (ELAIS). ELAIS was the largest single Open Time project conducted by ISO, mapping an area of 12 square degrees at 15mu with ISO-CAM and at 90mu with ISO-PHOT. Secondary surveys in other ISO bands were undertaken by the ELAIS team within the fields of the primary survey, with 6 square degrees being covered at 6.7mu and 1 square degree at 175mu. This paper discusses the goals of the project and the techniques employed in its construction, as well as presenting details of the observations carried out, the data from which are now in the public domain. We outline the ELAIS ``Preliminary Analysis which led to the detection of over 1000 sources from the 15 and 90 mu surveys (the majority selected at 15mu with a flux limit of ~3 mJy), to be fed into a ground-based follow-up campaign, as well as a programme of photometric observations of detected sources using both ISO-CAM and ISO-PHOT. We detail how the ELAIS survey complements other ISO surveys in terms of depth and areal coverage, and show that the extensive multi-wavelength coverage of the ELAIS fields resulting from our concerted and on-going follow-up programme has made these regions amongst the best studied areas of their size in the entire sky, and, therefore, natural targets for future surveys. This paper accompanies the release of extremely reliable sub-sets of the ``Preliminary Analysis products. Subsequent papers in this series will give further details of our data reduction techniques, reliability & completeness estimates and present the 15 and 90 nu number counts from the ``Preliminary Analysis, while a further series of papers will discuss in detail the results from the ELAIS ``Final Analysis, as well as from the follow-up programme.
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