A camera sensor is the part of the camera that captures light and converts it into an image. The size of the sensor determines how much light it can capture, and therefore how much detail and dynamic range the resulting image will have. In general, larger sensors capture more light and produce higher-quality images.
Here are some of the most common camera sensor sizes:
Full Frame
Full-frame sensors measure 36mm x 24mm, which is equivalent to the size of a 35mm film negative. Full-frame sensors are found in high-end DSLR and mirrorless cameras, and are preferred by many professional photographers for their image quality and wide dynamic range. Full-frame sensors also allow for shallow depth of field, which can be used creatively to isolate subjects from their backgrounds.
APS-C
APS-C sensors are smaller than full-frame sensors, but still larger than most other sensor sizes. They measure around 22mm x 15mm, and are commonly found in entry-level and mid-range DSLR and mirrorless cameras. APS-C sensors offer a good balance of image quality and affordability, and are popular among hobbyists and enthusiasts.
Micro Four Thirds
Micro Four Thirds (or MFT, or 4/3) sensors are smaller than APS-C sensors, but still larger than many other sensor sizes. They measure around 17mm x 13mm, and are commonly found in mirrorless cameras made by Olympus and Panasonic. Micro Four Thirds cameras are known for their compact size and high-quality lenses, and are popular among travel and street photographers.
1-inch
1-inch sensors are smaller than Micro Four Thirds sensors, but still larger than most compact camera sensors. They measure around 13mm x 9mm, and are found in high-end compact cameras and some drone cameras. 1-inch sensors offer good image quality and low-light performance, and are popular among enthusiasts who want a compact camera with high-quality optics.
1/1.7-inch
1/1.7-inch sensors are smaller than 1-inch sensors, but still larger than most smartphone sensors. They measure around 7.6mm x 5.7mm, and are found in high-end compact cameras like the Canon PowerShot G7 X Mark III. 1/1.7-inch sensors offer good image quality in a very small package, but are limited in their low-light performance and dynamic range.
1/2.3-inch
1/2.3-inch sensors are the smallest sensors commonly found in consumer cameras. They measure around 6.2mm x 4.6mm, and are found in entry-level compact cameras and most smartphone cameras. 1/2.3-inch sensors are very compact and affordable, but offer limited image quality, particularly in low-light conditions.
In addition to these common sensor sizes, there are also larger medium format sensors used in high-end cameras, and smaller sensors used in security cameras and other specialized applications.
With camera sensors, size isn't everything but, combined with the backend technology and image processor, sensor size is an important factor in deciding which camera is best for you.