Well, I guess the best way to describe it is that a succulent plant means they hold water more in than say a tropical plant (such as a 'Peace Lily').
This means they store water in their "bodies" (for a lack of a better word) and can go between waterings longer than than of a tropical plant. A lot of tropical plants grow where it is humid and rely on more frequent waterings to survive, whereas a succulent plant grows in areas where humidity and rainfall are less frequent (i.e. a the side of a mountain or in the desert).
Not to add to your confusion, but all cacti are succulents. However, not all succulents are cacti. To explain further, cacti are succulents, but they have spines (which are a type of modified leaf). Succulents that are not considered cacti, but just "succulents" have leaves.
Edited to say that there are spineless cacti. I know, confusing.
This is a picture of succulents for example
These are cacti
These are tropical plants. You see how their leaves are thin compared to the succulents? The succulents have thick, fleshy leaves designed to hold lots of water.
I hope this helps.
Planto
