Because there will be doubters.
When you tell people that you're an internet marketer or online entrepreneur, most will assume the worst. If they believe that you do make a decent income through your internet efforts, they will probably think that you engage in spam techniques or that you run some type of adult website or online gambling operation. They might figure that you just sell a lot of stuff on Ebay.
You can educate those who don't know while further developing your business in the process. Spammers are a small percentage of those individuals who make money online and because spamming is largely ineffective, those folks don't stay around long. Explain this fact to those who you believe need to be armed with accurate information.
Use your own email marketing as an example. Explain that each and every person that receives your email messages has signed up and provided their specific approval to be sent your email messages. Your subscription process is a lot like receiving a newspaper or magazine. Once an individual becomes a subscriber, they receive valuable information in addition to some advertising that is targeted to meet their needs and expose them to other opportunities that you recommend.
If you sell eBooks online, you can explain to people that you're a bit like a book publisher, but all of the books you publish are in digital format. You can tell them that instead of paying a lot of extra money for the printing, binding, marketing, and shipping of a physical book, you eliminate those costs by selling all of your books as downloadable products.
If people still don't understand, you could even show them what an eBook is by letting them view one on your computer. If your main business is advertising affiliate products, you can tell people you're in advertising.
Explain to them that you run a website (or websites) and you place advertising on that website. Every time someone clicks an ad and purchases something, you receive a commission. If you make most of your money through AdSense or selling advertising on your sites, you can simply tell people that you place advertisements on your websites, and you charge advertisers for that ad space.
If you want to, you could show them some of your sites or blogs to illustrate the point. Some people feel more comfortable just telling people they design websites. If you're a decent website builder, this will probably really impress people, but be prepared to get requests from people asking if you'll make them a site, too.
If you don't want to deal with all of the questions about spamming and porn, just telling someone you design websites will usually be enough to throw them off your trail. If people are particularly persistent with their questions, you can start to point out some examples of people who have been very successful making money online.
Don't just use examples of famous Internet marketers like John Reese or Yanik Silver! Use some examples with verifiable "proof" in mainstream media. For example, the famous "make money" blogger John Chow has a number of stories out there about him.
There's also teenaged entrepreneur Ashley Squalls who has made mainstream news in a big way by making over $70,000 per month with advertising on her site that gives away MySpace layouts. And of course, the infamous Perez Hilton - a celebrity blogger who cashes in on the ad space his blog has.
You should try to realize that some people will just never understand what you do. Try to explain it the best way you can, and if it doesn't work, accept it. Some people just won't get it. If the in-laws or your spouse are nagging you because your money isn't yet streaming in, and you feel pressured, put together a simple presentation that answers all of their questions and puts their minds at ease.
The more you can include people in your business or provide some ways for them to see and experience what you do, the more your work become legitimate and credible until the money begins to flow. Once you start living the internet lifestyle, you'll be surprised at how fast the questions go away.

Nice post, my man. I'm trying to figure out how to get more advertising on my blog, as it moves up the rankings and brings in more visitors all the time. I'm thinking that's going to end up being the way I'll make money, especially if I can keep putting out pretty good content.
Then again, I hope I'm not the only one who thinks I'm putting out pretty good content. lol
Posted by: Mitch | March 23, 2009 at 11:51 PM