How To Address A Cover Letter
When you're applying for a job, the best-case scenario is that you know who is supposed to receive and review your application. If that's the case, you should always address your cover letter to that individual by full name, first and last. You don't need to add in a relevant title if that's the case. "Dear John Doe," is just fine. Indeed, it's better to leave out titles since you don't want to make assumptions about gender. The name "Terry" could refer easily to a man or a woman, for example. What if you don't know the person's name though? How should you address your letter and ensure that it is polite and gets to the right person?
There are several acceptable greetings you can use. The majority of people use "Dear Hiring Manager." This is a good salutation for a couple of reasons. It isn't gender-specific, which eliminates that issue, and it also doesn't sound awkward. It's a simple, clear phrase. It also makes it obvious who you're trying to reach. You're looking to get your letter to the person who can give you a job. It clarifies the letter's purpose right off the top.
Another phrase that is commonly used is "To whom it may concern." There's nothing wrong with this phrase, although it is somewhat inferior to "Dear Hiring Manager." Why is it inferior? It's an awkward greeting. For one thing, while "whom" may be grammatically proper, how many of us actually use the word "whom" in conversation? For another thing, it isn't clear about your purpose. When you write "Dear Hiring Manager," that shows that you believe the Hiring Manager should be concerned about your letter. If you write "To whom it may concern," you're inviting ambiguity. What if it doesn't concern anybody? You've hardly made a case for anyone bothering with your letter. These are all subtle nuances. Again, you can use this phrase. It's just better to use "Dear Hiring Manager."
One more acceptable phrase is "Dear Sir or Madam." This phrase accounts for either gender, which is good, although it does sound awkward since it makes a big affair out of doing so. "Dear Hiring Manager" is a bit less ungainly in this sense. There is also something old fashioned sounding about saying "Dear Sir or Madam." You could look at this as a good thing (it shows you have proper manners and respect) or a bad thing (it could imply you're a bit outmoded). It's again a fine greeting, but you can see how "Dear Hiring Manager" might still be the better choice.
In terms of punctuation, it doesn't really matter what you use. A comma, a semi-colon or a colon is just fine; this bit is pretty irrelevant. With several good greetings to choose from, don't leave your greeting line blank. A blank greeting line communicates nothing, though it may make a hiring manager think that you're lazy, rude, or simply incompetent. If you can't make up your mind, always just default to "Dear Hiring Manager."