An extendable dining table has the obvious benefit over a standard dining table in that the size can easily be adjusted to meet your needs. It can be fully extended for a large dinner party or family gathering or can be reduced in size so it does not overwhelm your house for the rest of the time, making it a great table for smaller meals with the family or even for playing board games and doing homework on.
Before deciding on which one to buy, it is still really important that you check on the size of the room where you are planning to place it and compare this with the dimensions of the table when it is completely extended and at its smallest size. Some tables come with a set of alternative extension sizes which will mean that you will have a range of dimensions to consider. Check on the size not just of the table but with the chairs arranged as if in use and that means with space to get in and out, not just to sit.
The extensions for some extending dining tables are purely a piece of table top that can be placed in the centre of the table once it has been pulled apart. This of course will need storing when the dining table is not extended. Typically, small metal fastenings will clip the extension in place. Other mechanisms include opening out a fold out leaf or leaves of the table which are held in place by pulling across supporting legs or fold out supporting struts from underneath.
A novel idea, sometimes called a butterfly extension, involves pulling the table apart at the middle then lifting up and unfolding an extension that remains integral to the dining table when not in use and so does not require storage space. On a similar subject, you will always need to consider somewhere to store extra chairs when not in use.
Further options, such as the choice between a more expensive hardwood finish like walnut or going with a softwood dining table like pine, are discussed here http://everythingforyourfamily.com/