I just recently started to learn Japanese and I am a bit confused with some numbers. For example, what is the difference between 七 (なな) and 七つ (ななつ). Is there no difference? Is it just another way of saying it?
つ is a counter word (for generic inanimate objects), so that
七 (なな) "[the number] seven"
七つ (ななつ) "seven [objects]"
Similarly you have
七日 (なのか) "the seventh [day of the month]"
七本 (ななほん) "seven [longish objects]"
and many many more.
Depending on the counter, the numbers may not be based on the native Japanese numbers (hito, futa, mi, ..., nana, ...), but on the Sino-Japanese numbers (ichi, ni, san, ..., shichi, ...), as in
七時間 (しちじかん) "seven hours"
When the numbers (like 七) count something, you use a counter word. When you talk about the number as a number (e.g. in telephone numbers), you use the number without the counter word.