Cronbach alpha is Rather, alpha is a measure of level of mean intercorrelation weighted by variances, or a measure of mean intercorrelation for standardized data, stepped up for number of items.
Cronbach alpha based on standardized items is something similiar but is computed alpha when all scale items are standardized to have equal means and variances.
And that the regular Cronbach's alpha will usually but not always be less than or equal to standardized item alpha with both versions usually yielding the same substantive conclusions and both versions are widely used.
Some argue that the more restrictive standardized item alpha is appropriate when assuming classic parallel tests, otherwise conventional Cronbach's alpha should be used.