For a good introduction to the various methods that can be used to monitor soil water content for irrigation purposes, take a look at this slide show by Steve Evett of the USDA-Agricultural Research Service Conservation and Production Research Laboratory, Bushland, TX

Try the Soil Moisture Lecture notes slide show by Jerry Davis at the Dept of Geography & Human Environmental Studies, San Francisco State University

The lecture notes to the course SOILS 210: Introduction to Soil Science and Soil will also be of use as a starting point.

There are various methods of expressing soil water content (on IMKO's feature page.

Trime also has a page of mathematical definitions of soil water content, as well as a comparison of common moisture measurement methods. To convert between various units of measure, try convert-me.com or Maribi, or better still OnlineConversion.

Try this Introduction to Soil Water Potential

Estimating Generalized Soil-water Characteristics from Texture
    See also their amazing online Soil texture triangle hydraulic properties calculator

The article Soil Water relationships discusses water retention, and in particular, has an example graph of the water retention characteristic for a range of different textured soils.