std_logic values
As vinodkumar said, BIT has 2 values: '0' and '1'.
STD_LOGIC has 9 values: 'U', 'X', '0', '1', 'Z', 'W', 'L' ,'H' and '-'.
I know the meaning of some of them:
U = unknown (*)
X = collision - a multisource line is driven '0' and '1' simultaneously (*)
0 = logic 0 (*)
1 = logic 1 (*)
Z = high impedance (*)
W = (i don't know)
L = driven low weakly
H = drivel high weakly
- = (i don't know)
The values marked (*) are common y real logic simulation.
With BIT you use an ideal logic world, and with STD_LOGIC you have a more real behaviour of logic, and allows you to simulate tri-state signals.
I never heard about STD_ULOGIC.
The only difference I see between BITY and STD_LOGIC in synthesis is the 3-state generation.