A job offer is usually a package consisting of a position title, your base salary, medical and dental benefits, retirement benefits, vacation, medical and personal days, office hours, office location, supervisor etc. Any of these components can be negotiated though we dont recommend to negotiate all aspects at the same time. You should focus, first, on those components that influence whether you will accept the offer or not. You can also indicate that there are a few other aspects you would like to get an opportunity to discuss but indicate clearly that those will not influence your decision.
Ensure you have sufficient back up information before you initiate your negotiations. For example, if base salary in the offer is lower than your current job, you can be honest about it and discuss with your new employer if they may be willing to match.
Here is a list of things you can negotiate in a job offer. This list is not exhaustive.