An immigration attorney specializes in matters that have to do with immigration. This includes getting visas and residency permits, applying for citizenship, helping illegal aliens, etc.
You need an immigration lawyer if you are an illegal alien in the US, you want to get your green card, you want to apply for US citizenship, you have been served a notice of deportation, you want to ensure you file your immigration papers correctly or you need to get sound information on US residency.
An immigration attorney will help you by providing legal advice on all immigration matters that you might be faced with, help you stay up to date with constantly changing immigration laws and help you ensure every filing is done correctly. This type of lawyer will also help you build a solid case by getting evidence that support your case and help you file an appeal if your application is declined.