A secondary cell is a cell that is designed to be recharged with electricity and reused many times. In general, the electrochemical reaction occurring in the cell is reversible, and so these cells can be recharged. Secondary cell works Faradays laws of electrolysis.
Examples of secondary cells include a lead-acid battery, nickel-cadmium (Ni-Cd), nickel metal hydride (Ni-MH), lithium-ion (Li-ion), etc.

A primary cell is a cell that is designed to be used once and discarded, and not recharged with electricity and reused like a secondary cell. In general, the electrochemical reaction occurring in the cell is not reversible, and so these cells cannot be recharged.
Examples of primary Cells are dry Cells, Daniel Cells, etc.

Both primary and secondary cells are useful for various applications. The primary cell does not reverse the chemical reaction after discharging. Therefore, it can not be reused. On the other hand, the secondary cell reverses the chemical reaction after discharging. Therefore, we can use it over & over again till its useful life exhaust.

The capacity of batteries is typically specified in terms of the total amount of electrical charge it can store. The unit of measurement for battery capacity is usually either ampere-hours (Ah) or milliampere-hours (mAh). Here's a brief explanation of these units:

Ampere-Hours (Ah):

• One ampere-hour is equal to the flow of one ampere of current for one hour.
• It is a larger unit and is commonly used for larger batteries, such as car batteries or deep-cycle batteries.

Milliampere-Hours (mAh):

• One milliampere-hour is equal to the flow of one milliampere of current for one hour.
• It is a smaller unit and is commonly used for smaller batteries, such as those in smartphones, cameras, or other portable electronic devices.

Battery capacity (AH) is defined as a product of the current that is drawn from the battery while the battery is able to supply the load until its voltage is dropped to lower than a certain value for each cell. Capacity of batteries usually expressed in Ampere Hour (AH).