Specially Quadcore ones for mobiles and which one is best. How to understand.
If possible please, which device has under 25K INR or 400$
ARM Cortex is a series of reference microarchitectures released by ARM Holdings PLC that span the ARMv6, ARMv7, and ARMv8 families.
The ARM Cortex A series are meant for applications and are often found in mobile phones
The ARM Cortex R series are meant for real time applications and are found in signal processors
The ARM Cortex M series are meant for microcontroller applications and are found in control systems often alongside R series devices.
ARM Holdings does not manufacture any devices themselves. Manufacturers such as Apple, AMD, NVidia, Qualcomm, and Samsung licence the designs in a synthesizeable format that can be integrated with their own designs.
Some manufactures may go a step further and customize the designs to their choosing or even design their own microarchitecture that is compatible with a particular instruction set. For example, the Qualcomm Krait is a series of ARMv7 compatible microarchitectures that is similar (if not directly derived from) the ARM Cortex A15.
I myself use a Samsung Galaxy S4. The North American edition has a Qualcomm Snapdragon 600 microprocessor which employs four Krait cores. The international edition has a custom Samsung Exynos 5 microprocessor which employs four Cortex-A15 and four Cortex-A7 in a power balanced configuration.