What is a classification? It is basically the "category" that your trademark will fall under. For example, let's say you have a t-shirt company called Hal's T-Shirts.  You will most likely want to trademark Hal's T-Shirts under the category of "T-Shirts" to start.  As you type in the search term "T-Shirts", the US Identification of Goods and Services Manual (http://tess2.uspto.gov/netahtml/tidm.html), will (hopefully - we will get to the story later where you type in a search term and absolutely nothing that is accurate will come up) turn up the results which will let you know which number is assigned to your classification of your trademark, (T-Shirts happens to be the number 25)

Each trademark classification, if you are filing a regular application, costs $325.00. So, it's probably not a good idea to pick as many classifications as you can. The goal is to accurately pick the classifications that truly fit your company.  As your company grows, you can add more.  No need to add "egg incubators" when your company has no intention of ever selling those services.  And yes, I did actually find "egg incubators" when I was doing a trademark search one day.

I am going to do more posts on trademark but for now I just wanted to address the classification issue. Don't feel pressured to file as many trademarks as possible so you can feel "protected". The goal is to be strategic and really look at your lines of business, what you currently do and where your company will grow in the future. Build your trademark portfolio around those concepts and you are on your way to a successful trademark mission!