The standard deviation tells us how spread out the data is from the mean. It is the square root of the variance.
Mathfun site describes the SD and variance in very simplest way with the example of height of 5 dogs.
Robert Niles explains the concept of the standard deviation from his stats guide series.
Resource page on standard deviation.
A pictorial guide on how to calculate the standard deviation by wikihow.
A series of informative, detailed and interesting lectures on standard deviation hosted on Khanacademy website.
Step by step picture guide to calculate the mean and standard deviation with MS Excel.
Defines the 2SD and 3SD hosted on University of Surrey site.
Summarizes the 6 steps for calculating the standard deviation.
Learn the SD concept through interactive game. To run the game, Java should be installed on your device. Page hosted at University of Glasgow.
This BMJ article discusses the relationship between standard error and standard deviation. Standard error is the measure of the precision of sample mean and depends/influenced by the standard deviation and sample size.
A detailed article with examples.