First, don’t have conversations in cubicles that need to be private or strictly confidential. Most cubicle walls do not extend all the way to the ceiling and even if they did, most cubicles don’t have doors either let alone any soundproofing. Any such private or strictly confidential conversations should take place elsewhere depending with whom you are talking: in your car, in a conference room, or outside the building during lunch break. Likewise, even if you are having a conversation suited for those near your cubicles, sound travels surprisingly well in most offices, so try not to be too loud that it interrupts others who are working.
Sometimes there are multiple conversations going on at once or multiple employees taking part in one conversation. The etiquette on when/where/how to join such conversations is so variable depending on the situation that it’s impossible to pin it down here. Let’s just say that it’s helpful to remember, especially for very social or inquisitive employees, that most people don’t like an employee who tries to jump into every conversation all of the time, let alone constantly makes other employees repeat other parts conversations. A good rule of thumb is that if another employee specifically asks you a question, you are “officially” invited into the conversation.
Lastly the topics of cubicle conversations are important to keep professional. Again, the etiquette of which topics are professional, in which companies/industries, and to what extent, are impossible to pin down here. Adult/explicit, gruesome/violent, and illegal actions are some examples of non-professional topics in any given workplace. When in doubt, fall back on some common sense. How the topics are discussed also contributes to professionalism. Employees who conduct themselves the most professionally generally don’t argue violently or savagely gossip about others behind their backs, for example. Having respect for other people in the conversation, or the subjects of the conversations, goes a long way to helping keeping cubicle conversations professional.