SmartPhones are a new breed of phones which are changing the way we communicate at an astonishing speed.

As per wikipedia,

A smartphone is a mobile phone that offers more advanced computing ability and connectivity than a contemporary feature phone. Smartphones and feature phones may be thought of as handheld computers integrated with a mobile telephone, but while most feature phones are able to run applications based on platforms such as Java ME, a smartphone allows the user to run and multitask applications that are native to the underlying hardware. Smartphones run complete operating system software providing a platform for application developers. Thus, they combine the functions of a camera phone and a personal digital assistant (PDA).

If you are also thinking of making a trasition from your old mobile phone to a SmartPhone, then here is a list of features which i think are a must for a smartPhone to be a real smartPhone:

	WiFi No strings attached!! This is the mantra today to connect any device to internet. Your smartphone should support WiFi connectivity. These days, you will find free or paid WiFi networks in most cafe's, hotel lounges and other social places. Just switch on WiFi and connect online in seconds.
	Battery Back Up As you would be doing lot of work like internet surfing, chatting, listening to music, watching videos and last but not the least calling and receiving calls. Also, do not just blindly believe on the battery talk time written on the phone specifications. Most of the time, these are the best case scenarios where the company has just tested the battery back up in a scenario where the phone was just kept in idle mode with no activity. But would you ever keep your smartPhone without using its features? In real world, manufacturers do install inferior quality batteries to keep the cost of the mobile low. So, I would suggest reading the real battery reviews for the smartphone model that you looking for. You would find many forums in which people would have complained or appreciated about the various features of the phone along with battery.
	RAM Memory This is an important part of any smartphone. If you do not have sufficient RAM, you wil not be able to run multiple applications. It should at least be 512 MB as per current standards. Off course this will change quickly as RAM gets cheaper with increasing smartPhone base.
	USB connectivity This is one of the mandatory feature which should form part of any smartphone. USB is a standard these days to connect to any other electronic device like a laptop or a digital camera. Smartphones generally do not have the USB of the same size as a laptop but they do carry a Micro USB and a converter cable to connect to laptop USB port. Also, when you connect your SmartPhone and your Laptop with this Micro USB to USB connector, the phone's battery starts charging itself too!! So, that is another great feature supported in many smartPhones today.
	3G Even if you do not plan to use 3G in near future due to high cost of usage currently, you should ask for a 3G enabled device. Keeping the pace of enhancements in mobile technology in mind, the day is not too far when 3G will also be cheap and you would then be able to access high speed inetrnet on the go itself. Remember, who could imaging that a call would only cost 1/2 paisa (cent) per minute 10 years ago?
	GPS Another important feature that is gaining relevance in Indian market. GPS is a global positioning system which helps you find directions using your latitute and longitude position on earth map. It is highly used feature in GPS, smartphones and cars in US and other developed countries where the population is low and you can't find people to show you the way. Even though you can find people in the remote areas of India too to ask for directions, this feature is highly useful and reliable.
	Good OS Your experience on using a smartphone will largely be defined by the OS (Operating system) it is running on. Google's Android is one the best mobile oS currently in market. It is an open source OS with a large base of application developers. So, it is a good idea to look in for Android enabled smartPhone and expose yourself to thousands of Android applications.

These are the features which make a phone a SmartPhone. If they are missing, you are better off buying a simple phone.