Though BGM (Blood Glucose Monitoring) and CGM (Continuous Glucose Monitoring) provide information about your glucose levels, there are some key differences:

Measuring frequency
The frequency of obtaining glucose values is different between using a BGM and CGM. Using a BGM, you might check your blood glucose levels several times a day, typically before meals and bedtime, depending on your type of diabetes and medication.

With CGM, your glucose level is measured continuously. The glucose values measured by the sensor in your skin are transmitted every 5 minutes and combined to create a glucose curve. You get a full picture of your glucose levels over the time you are wearing the sensor.

Measuring glucose levels in blood vs interstitial fluid
BGM and CGM also differ in where the glucose values are measured. BGM measures glucose in capillary blood drawn from your fingertips (or sometimes other places like the palm of your hand). The sensor of the CGM solution, in contrast, is placed in your skin, more specifically, in the fat layer below the skin (subcutaneous fat tissue). There, your glucose level is measured in the fluid between the cells (interstitial fluid, not blood).