And operator will check all conditions in the statement before continuing, whereas the Andalso operator will stop if it knows the condition is false. For example:
if x = 5 And y = 7
Checks if x is equal to 5, and if y is equal to 7, then continues if both are true.
if x = 5 Andalso y = 7
Checks if x is equal to 5. If it's not, it doesn't check if y is 7, because it knows that the condition is false already. (This is also called short-circuiting)
Source: http://stackoverflow.com/questions/302047/what-is-the-difference-between-and-and-andalso-in-vb-net
