AAA ; 37 
AAS ; 3F 
AAD ; D5 0A 
AAD imm ; D5 ib 
AAM ; D4 0A 
AAM imm ; D4 ib 
These instructions are used in conjunction with the add, subtract, multiply and divide instructions to perform binary-coded decimal arithmetic in (one BCD digit per byte - easy to translate to and from ASCII, hence the instruction names) form. There are also packed BCD instructions DAA and DAS: see DAA.
AAA (ASCII Adjust After Addition) should be used after a one-byte ADD instruction whose destination was the AL register: by means of examining the value in the low nibble of AL and also the auxiliary carry flag AF, it determines whether the addition has overflowed, and adjusts it (and sets the carry flag) if so. You can add long BCD strings together by doing ADD/AAA on the low digits, then doing ADC/AAA on each subsequent digit.
AAS (ASCII Adjust AL After Subtraction) works similarly to AAA, but is for use after SUB instructions rather than ADD.
AAM (ASCII Adjust AX After Multiply) is for use after you have multiplied two decimal digits together and left the result in AL: it divides AL by ten and stores the quotient in AH, leaving the remainder in AL. The divisor 10 can be changed by specifying an operand to the instruction: a particularly handy use of this is AAM 16, causing the two nibbles in AL to be separated into AH and AL.
AAD (ASCII Adjust AX Before Division) performs the inverse operation to AAM: it multiplies AH by ten, adds it to AL, and sets AH to zero. Again, the multiplier 10 can be changed.