Not all customers are alike. What appeals to one may not interest another. It's important that you connect your customers' different interests and needs to your message. It's about relevance. Relevant emails are opened, irrelevant emails are unopened or deleted, resulting in a lost connection and a lost sale.
Every small business can segment their customer base at some basic level. Think about what makes sense for your business. When you approach new customers, what are the qualifying questions you ask them? For retailers, it may be as simple as which consumer buys what product line. For B2B, it may be more about where that potential business customer goes to get their services. Segments are the things you keep track of when you're qualifying leads and identifying prospects. It's up to you to identify them. Trust your instincts, then divide your list into two major groups.
Segments should be based on your overall email marketing plans and what you want to communicate about your business. By tapping the right customer's passion and need at the right time--with a targeted subject line and targeted content--and you're much more likely to create a sale.
Source: Constant Contact