Simple segmentation tips that will help you figure out your subscribers:

10/ By demographics

Most subscribers can be usefully divided by age, gender, location, position in company, etc. These are more or less static criteria and define a customer with a very broad brush. If you are just starting out in email marketing then such descriptions can be useful.

9/ How they subscribed

There is every possibility that someone who makes their way around trade fairs will have a different outlook to those you garnered from a counter promotion. 

8/ Purchasing history

A subscriber who has just bought an item can often be tempted to buy another fairly soon afterwards. Those who haven’t bought anything for a while might be worth exciting by way of a generous offer.

7/ Reaction to triggers

It is a well known phenomenon for bicycle sales to increase at the time of the Tour de France. If there is a peak in completions after an email marketing campaign, work out what caused it and who responded.

6/ How much they have in common with others

The main trick with email marketing lists is to put those who respond in a certain way to a campaign into their own list with the same design of email, sending time, and mode of address. 

5/ Website visits

Subscribers who frequent specific pages on your website might have other features in common. Also, if they surf a bit around 3.30 on Thursday afternoon, this might indicate a good time to send.

4/ Scores on the metrics

Those with a common and consistent score on, for instance, click throughs, might well have other things in common.

3/ What they have bought

A printer bought a few months ago might well need consumables after three months. 
 
2/ Time on your list

You subscribers will tend to follow a certain life cycle. Those on the main sequence might well be too large a group for their own list, but those who fall outside in various ways could be usefully segregated.
  
1/ Gut feeling

Experience costs and it is only right that you should exploit it. If you think a certain classification of subscriber might respond well to a particular email marketing campaign, then test your instinct.