The DV Green Card lottery is a program by the US state department, which gives 50,000-55,000 immigrant visas per year to qualified applicants. The applicants are randomly drawn in a mail in lottery.
Participation is absolutely free, simple and no special entry form is required.
Most green card lottery related sites offer help in filling out and submitting entries for a fee, these are listed in the Submission_Services sub-category. Participation doesn't require special skills (beyond reading and writing)