Scientists used to lump protists into a single kingdom, and they still use this classification for some purposes. However, science has largely recognized that what we call Kingdom Protista actually includes a wide range of organisms that are not particularly related. Biologists are currently in the process of revising their classification to reflect the evolutionary relationship among this huge group of organisms.
The protists exhibit considerable diversity. Almost every rule used to describe them has some major exception. Some protists live as parasites, others as predators and still others as producers. Some have rigid cell walls, while others have more flexible cell membranes. Their methods of movement include passive drifiting, swimming with flagella, swimming with cilia and creeping along with pseudopods. Even certain very basic criteria that have been used to define the group, such as the presence of nuclei and mitochondria, either don't exist or take on bizarre forms in some protists.
Scientists have tried to classify the organisms within the protists as either plant-like, fungus-like, or animal-like. However, genetic testing and close examination have revealed that these categories often don't hold up, either. For example, Euglena have characteristics of both plant-like and animal-like protists. Euglena have chloroplasts like plants--that is, structures that allow them to gain energy from the sun through photosynthesis. At the same time, they have a tail or flagella which they use to swim, making them mobile, a very animal-like characteristic. Many other protists also have characteristics that makes it very hard to justify keeping them all in a single group or subgroup.
Attempts at Sorting
Scientists have begun to sort protists into different groups. In fact, the protists can be sorted in three to ten proposed kingdoms -- different researchers divide them differently. Scientists are trying to create these groups based on evolutionary relationships. The goal in forming kingdoms is to group all the descendants from a common ancestor into one group.
General Characteristics -- No Perfect Rule
Treating the protists as a single group can work in certain areas of biology. For example, in medicine it might not matter which specific protist kingdom is causing an infection if the treatment for all protists is the same. The only real constant rule is that all protists are eukaryotes, meaning that they are organisms with more complex cells than bacteria. Most protists have a single independent cell, though some seaweeds break this rule. Most protists have a single defined nucleus holding the bulk of their DNA, though some ciliates have multiple nuclei and flagellates do not have a defined nucleus. As a group, it is hard to find a definition that describes all protists perfectly.