You will not receive any tax forms for your contributions, as the deadline for contributions is the due date of your tax return. However, any contributions will be reported on the Form 5498. This form is reported directly to the IRS by Betterment and will be uploaded to your account for reference.
You will receive a Form 1099-R if you made any distributions at tax time.
Betterment is not a tax advisor, nor should any information herein be considered tax advice. Please consult a qualified tax professional.