first grade science

That is a funny question . Apart from appearance not much difference. They are both plants which have adapted to their environment in the most suitable way. Very few trees could live in a Desert because they would soon run out of water. A Cactus has built up a protective coat and usually avoids spreading out leaves that lose water quickly. A tree has developed a method of over topping other plants and shrubs tpo get maximum light . Some Cacti grow really tall and resemble trees with very fat trunks.
Youre question has made me think hard. I look forward other peoples explanations

quianda, A cactus has it's own plant family called Cactaceae, a group of spiny type plants. They are part of a larger group of plants called Xerophytes. All plants in this group have adapted to an environment which offers very little water in any given year. There are traditional looking shrubs and trees which belong to this group. Succulents belong to this group and a cactus is a succulent but not all succulents are cacti; therefore it has its own family. A cactus is a flowering plant (Angiospemae), as well as a seed bearing plant (Spermatophyte)

Some cacti have the same parts as a tree and some meet the criteria for a shrub but those parts cannot be easily identified by the average person because of the extreme difference in their appearance and location in the world due to their adaptation to the extreme elements. That is why they have been categorized into their own family. A cactus is a cactus and would never be referred to as a type of bush or tree when being identified on the open desert.

This is probably more than you needed to know but I hope it helps.