A paragraph is a distinct section of writing covering one topic. A paragraph will usually contain more than one sentence.
A paragraph starts on a new line. Sometimes, paragraphs are indented or numbered. (Whatever format you use, be consistent.)
The "perfect paragraph" will start with a topic sentence. It will have detail sentences in the middle and end with a concluding sentence. It will only cover one topic from start to finish. The length of a paragraph is supposed to be determined by the topic, but often writers will create a paragraph simply to ensure they're not presenting too much text in one chunk.
A paragraph could be part of a text that informs people, describes something, critiques something, compares things, persuades people, lists a process, makes an argument, offers a solution or narrates a story. And, the level of detail will vary from text to text.
All this diversity means that it's not always easy to determine what "one topic" means when dividing your text into paragraphs. For example, you could have a one-topic paragraph describing Venus (with the next paragraph describing Mars) or a one-topic paragraph describing the colours of a sunset (with the next paragraph describing its reflection in the sea).
If you're getting the sense that the word topic is a bit too grand for a measly paragraph, then think of a paragraph as a distinct section of writing that covers one aspect of your topic. That's the point. Sometimes, a paragraph will be an aspect of a topic, sometimes it will be a topic within an issue, sometimes it will an issue within an argument…a narrative, a process, a comparison, whatever. Whatever the scope of your paragraph, it should be neatly bounded as one…well, topic. If you prefer aspect instead of topic, go with that.