Symbiotic relationships are the relationships organisms in a community have with one another. Types of symbiotic relationships includes mutualism, parasitism, predation and commensalism. In mutualism both organisms benefit, in commensalism one organism benefiting parasitism one organism benefits, in predation one organism benefits while the other dies.
One of the standard kind of relationships that ecologists will study between 2 species in a community is something called a symbiotic relationship. Symbiotic relationships are when you have 2 organisms that live in close context some or all of their lives. Now stereotypically, just mutualism and commensalisms some people will also include parasitism and I've even seen some tests or text books that include predation. So I'll describe all of them so we can see how they all inter relate. Mutualism is basically when 2 different creatures both benefit from their relationship, there's many examples of this in nature. For example cats and humans, cats get nice shelter, we feed them. What do humans get? Well they hunt down rodents for us and in modern times they've actually found that having a cat helps reduce the amount of negative emotions that people have similarly between dogs and humans.
Other examples in nature might include liking as a common example given where you have a fungus and an algae actually working together. The algae provides glucose fro photosynthesis while the fungus provides water and minerals from the rock that's able to break down for the algae. Now with commensualism you end up having one organism benefit without hurting or helping the other. Some common examples of this might include for example birds nesting in a tree they get the benefit of protection from predators up in the tree and this doesn't really hurt the tree and it doesn't help it.
Now parasitism that's where one benefits and the other is hurt, there's lots of examples that you can think of for parasites. Like tapeworm and roundworms and hookwarms lots of different kinds of warms act as parasites. Now again predation is not commonly thought of as a form of symbiosis but you hopefully know what that is, that's where one benefits because it killed the other and ate and that's symbiosis.