I will try to explain the process in very basic steps
On a very highlevel, there are three steps for Employment based green card:
1. Labor Certification
2. I 140
3. I 485
Even before starting the labor certification, I call it the "Step zero", you have to convince your employer (basically, manager) to start your green card process. Typically, this will involve, you talking to the lawyer and starting to fill the forms/questionnarie required. The equal amount of forms/questionnarie needs to be filled out by your manager as well. Then, the job posting for you which you have applied for, should be posted online, intranet as well as local news paper (3 types of ads). If your employer gets applications for these job postings, they need to document each applicant and if required do interviews. Then, they have to justify why no other US Worker fits this position.
Once this step zero is done, the lawyer files for the labor certification process electronically. After the labor certification application (ETA Form 9089) is approved, the lawyer files the next step, I 140. Once the I 140 is approved, you can file for I 485. When your I 485 is approved, you get yoru green card.
The problem is, now few of us cannot even apply for I 485. For countries like India and China, there are lot of applicants. But, there is a specific quota for country-category combination. There is a serious backlog, so I 485 can be filed only when the category/country is current or you reach the 'priority date' -- simply put the waiting line number. Priority date -- is the date on which Department of Labor acknowledges that the LCA is filed.
Disclaimer: I am not a lawyer. Its always better to do the green card process through an immigration lawyer and consult specifically on your case.