What is Star Trek all about?

Here are some frequently asked questions from people who aren't very familiar with Star Trek. 

Well, there are a few ways this can be answered. 

1. Budget for Practical Effects
   It's important to remember that TV shows, especially those in the 1960s, had a very tight budget. The '80s and '90s allowed for more of a budget and the advancement of practical effect technology and techniques allowed for much more unique prosthetics, makeup, costumes, etc. We now live in a time where characters can be entirely rendered within a computer, their movements brought to life through motion-capture technology, and their faces and voices synched—but these were not even close to being options for the time, especially on a television series budget.
   As time has gone on, budgets have increased and some alien species that were introduced decades before look much different (like the Klingons). Some people are pleased by this, others are not. 

2. Acting, Interaction, and Engagement
   Another reason is to allow the actors to, well, act! While the vast majority of alien characters depicted in the series are bipedal and mostly humanoid in various ways (enough to allow an actor to wear a costume to portray them), there are quite a few exceptions. However, these exceptions are often limiting in the ways that the alien and the cast are able to interact. For example, there are many episodes that feature large, space-dwelling alien lifeforms that may or may not be intelligent creatures, and while they might be very cool to look at and to consider the ways in which they are different from or similar to us, they are very rarely a character that can actively speak let alone provide an engaging performance and dialogue. 

3. Plot, Themes, and Social Commentary
   The nature of many story plots in Star Trek revolve around how we as humans in a fictional future interact with other races who share this galaxy (and beyond) with us. This often involves attempting to communicate, negotiate, and express motivations and the right to exist, from an individual level to entire species. In a lot of ways, these alien races are just extensions of us humans, in a thematic sense, in a similar way that people from different nations on Earth go about handling such things. Star Trek offers even more possibilities to explore ourselves and "what if" scenarios, examining humanity from an outsider's perspective. Many stories are much more meaningful to us when we can actively relate to them, and so having actual people portraying members of alien races can help provide the ground for the empathy that is required for us to see ourselves in them and then bring that around to how we see these themes reflected in our real lives.  

4. In-Universe Explanations
   Believe it or not, there's actually an episode that attempts to address this. I won't spoil it any further, but if you are curious, check out Star Trek: The Next Generation season 6 episode 20: "The Chase."