All mobile phones contain some memory. Many allow the addition of external memory cards to increase that memory.
The internal memory is often split on those phones that have a large amount [see the Nokia N96 and X6] the split allocates a percentage of the memory into two areas like the hard drives on a computer.
The main memory will be shared by any applications you install and you Contact list, Messages, alerts, Calender, notes etc.
The other memory is usually a storage area accessed by the music apps and photo apps.
In order to know exaclty how many photos you can store you also need to have some idea how much memory is left over after to install your choices.
The 2 Nokias I metioned both have larger than average internal memory. You should find there is plenty of room compared to any older phones you may be used to.
Photos are stored at different resolutions too. You will need more space if you store pics at maximum resolution and the camera is a larger Megapixel count than the camera you are used to.