Form W-2

Your employer will combine the amount of any reimbursement or other expense allowance paid to you under a nonaccountable plan with your wages, salary, or other pay and report the total in box 1 of your Form W-2.
You can deduct your expenses regardless of whether they are more than, less than, or equal to your reimbursement.
Reimbursements you received for nondeductible expenses are treated as paid under a nonaccountable plan. You must include them in your income.
You must include in your income reimbursements your employer gave you for expenses of education that you need to meet the minimum educational requirements for your job, or is part of a program of study that can qualify you for a new trade or business.

If you are reimbursed under an accountable plan, your employer should Not include any reimbursement in your income in box 1 of your Form W-2.
To be an accountable plan, your employer's reimbursement arrangement must require you to meet all three of the following rules.
Your expenses must have a business connection—that is, your expenses must be deductible under the rules for qualifying work-related education.
You must adequately account to your employer for your expenses within a reasonable period of time.
You must return any reimbursement or allowance in excess of the expenses accounted for within a reasonable period of time.
If your expenses are more than your reimbursement, you can deduct your excess expenses on Form 2106.

Generally, Payments made under an accountable plan are excluded from employee's gross income and not reported on Form W-2, However
if the per diem or mileage allowance exceed the IRS rules the excess is treated as wages in Box 1 and also in Box 12 Using Code L.
These amounts are subject to Social Security, Medicare and Withholding taxes.

The amount in Box 1 can be unlimited but the amount in Box 3 and 7 should never exceed $ 106,800 in 2011. Also;
Elective salary deferrals to a 401(k), 403(b) Simple, Salary reduction SEP as well as payments of adoption expenses and deferred amounts under a nonqualified plan or 457 plan are included in Box 3 and not in Box 1 Taxable wages.

If you worked in a restaurant employing 10 or more people, your employer will report in Box 8 your share of taxable tip income.
The amount in Box 8 is includable in Form 1040 Line 7.

Elective deferrals from retirement plans and adjustments or deductions are listed with a code and an amount in Box 12.
CODES:
A = Uncollected SS or RRTA tax on tips
B = Uncollected Medicare tax on tips
C = Taxable cost of group life insurance over 50K
D through H,S,AA,BB,EE = Elective Deferrals and Roth contributions
J = Nontaxable sick pay
K = 20% excise tax on golden paachute payments
L = Employee business expense reimbursements
M = Uncollected SS or RRTA tax on life insurance
N = Uncollected Medicare tax on life insurance
P =excludable moving expense reimbursements
Q =Nontaxable combat pay
R = Employer contributions to Archer MSA
S = salary reduction under 408(p) Simple
T = Adoption benefits
V= income from stock option
W = HSA employer contribution
Y = 409A deferral
Z = 409A income
AA = Roth contribution under 401(k)
BB =Roth contribution under 403(b)
DD = employer health coverage
EE =Roth contribution under 457(b)

If the recipient is a statutory employee, wage income and expenses are reportable on Schedule C. Earnings are not subject to income tax withholding but are subject to SS and Medicare taxes.
Covered by a retirement plan box being X'd means the recipient is subject to the phase-out rules for IRA deductions.
Income reportable as 3rd party sick pay has this checkbox X'd.

Taxable fringe benefits and miscellaneous payments are listed in box 14.

If, even after nudging from the IRS, your employer doesn't send you a replacement W-2 in time for you to file your return, you may file using Form 4852 in place of your missing wage statement.
After you complete the form, attach it to your tax return in place of your absent W-2. A copy of Form 4852 also should satisfy your state tax collector for those returns.
Using Form 4852 instead of an original W-2 may delay your refund while the IRS verifies the information you provided.
In cases where an employer has filed for bankruptcy or ceased operations, the IRS suggests you send a copy of Form 4852 to your local Social Security Administration office.
This should ensure that you get proper credit for the Social Security and Medicare taxes you paid so that your checks will be correct when it comes time to collect these benefits.