This is a simple 3.5mm TRS stereo audio jack with a screw-on plastic shroud. These are great for projects that plug into headphone jacks. Stereo plugs can also be used for carrying serial comm connections!
You may recognize these as the connectors that are commonly used for home stereo speakers. They happen to make good spring terminals for most applications! There are 4 separate terminals in all with two mounting holes for panel mounting.
This is a simple breakout board for the MQ - 3, MQ - 4, MQ - 6, MQ - 7, and MQ - 8 gas sensors. With the Gas Sensor Breakout you will be able to utilize each oversized, oddly spaced pins featured on every MQ gas sensor product. This is an extremely handy and economical board to have around when detecting multiple types of gases.
This is a 3.5mm TRRS SMD audio jack. TRRS connectors are the audio-style connectors that you see on some phones, MP3 players and development boards. TRRS stands for "tip, ring, ring, sleeve," which reflects the fact that, unlike a standard stereo connector, this actually has three conductors and a ground. Some devices use the extra conductor for a microphone (like hands-free headsets) or to carry a video signal (like in some MP3/MP4 players).
This is the kind of 1/4" stereo jack you might find in home stereos and PA systems. Switched contacts allow you to detect whether or not there is something plugged in to the jack. The jack can be panel mounted in an 11mm diameter hole.