Dependent samples are either two groups matched on some variable (for example, age) or are the same people being tested twice (called repeated measures).
http://en.wikipedia.org/wiki/Student's_t-test

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Note added at 1 sat (2008-08-27 23:00:31 GMT)
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Dependent samples test. The t-test for dependent samples can be used to analyze designs in which the within-group variation (normally contributing to the error of the measurement) can be easily identified and excluded from the analysis. Specifically, if the two groups of measurements (that are to be compared) are based on the same sample of observation units (e.g., subjects) that were tested twice (e.g., before and after a treatment), then a considerable part of the within-group variation in both groups of scores can be attributed to the initial individual differences between the observations and thus accounted for (i.e., subtracted from the error). This, in turn, increases the sensitivity of the design.
http://www.statsoft.com/textbook/glost.html

Difference Between Proportions : Dependent Samples
http://www.stat.wmich.edu/s160/book/node64.html