The simple definition of synchronous is synchronized while asynchronous, therefore, is not synchronized.

In distance learning, these terms relate to how classroom time is spent. In the synchronous classroom there are times that all the learners are required to be logged in together, at the same time (for example, during live classroom chats). Even in a synchronous classroom most of the student's time is spent in asynchronous activities.

In the asynchronous classroom, there are no expectations that all the students will come together at the same time, but rather they can access the classroom at any time, and turn in assignments or post responses to discussion questions at their own pace, but by set deadlines.

There are advantages and disadvantages to both types of online classrooms. While synchronous events like live chats, presentations, and discussions, can help bring cohesion and connection to a classroom, they may also disrupt the lives of the learners. Most online learners are adults with jobs, families, and commitments outside the classroom. In addition, online learners are rarely in a single time zone. Attending live, online events can be difficult. On the upside, it's a common fear and reality that online students can feel separate and alone from their classmates. Having synchronous events lets all the students know that their classmates and the course facilitator are real people.

Asynchronous classrooms are perfect for classes where learners are spread out around the world. They allow busy adult learners to build their own schedules and meet classroom commitments interspersed with their work and home life commitments. Asynchronous learning environments work well in the online setting and they're also the main reason many adult learners return to academic settings.

I personally prefer wholly asynchronous settings. I like the freedom they provide in my schedule, both when facilitating such classrooms, and also when attending them as a learner. But, I also like having occasional opportunities for live interaction.

Hybrid online settings are becoming more common. Those that have a primarily asynchronous set up, but occasionally include a synchronous event. These live events usually aren't a required or graded portion of the class, but rather an opportunity for online learners to come together if they can. It's important to remember to record these events and upload the recordings or transcripts to the classroom so those who couldn't attend can still see what happened.

If you're an online classroom facilitator, one way to introduce a bit of synchronous activity into an asynchronous classroom is to provide office hours in the live chat setting. Chat rooms happen in real time, giving that conversational feel. You can also generate a transcript from these chats which can become the core of a frequently asked questions (FAQ) section for your classroom.