It's the most important aspect of a laptop that determines how the interface looks, how it works and the types of software it supports.

Windows: The longest running, most flexible and widely adopted operating system. Its latest version is Windows 10 which is easy to navigate, offers great security and includes applications which make your daily tasks easier.
Windows OS
MacOS: Apple's own Operating System designed specifically for Macbooks. MacOS High Sierra is the latest version which is very stable, secure and boasts an elagant and easy-to-use user interface.
Mac OS
Chrome OS: Designed for Chromebooks, Chrome OS is fast, simple and efficient. It is great for social media usage, handling small tasks, and provides access to the Google Play Store as well.
Chrome OS
DOS: DOS is the default operating system which you get with certain laptops as compared to a pre-installed OS such as Windows. It's good to get a laptop with DOS if you plan on installing a different OS altogether.
DOS OS

The internal storage determines the amount of data (photos, videos, music, software etc.) that can be stored in your laptop.

Hard Disk Drives (HDD): The most common type of internal storage which is ideal for storing large amounts of data such as movies, games, etc.
500 GB 1 TB 2 TB
Solid State Drive (SSD): SSDs (also known as Flash Storage in Macbooks) offer much faster access to files but lesser storage capacity which makes them ideal for people who look for high performance computing irrespective of the storage space.
128 GB 256 GB 512 GB 1 TB
Embedded Multimedia Card (eMMC): EMMC is a flash-based storage used in inexpensive laptops. Usually the average storage capacity of these laptops is around 32-64 GB while performance wise, they are slower than SSDs.
eMMC

RAM (Random Access Memory) determines how smoothly and efficiently your laptop runs. More the RAM, the better your Laptop is at multitasking.

4 GB: Suitable for simple browsing(limited tabs), streaming basic college/office work, etc.

8 GB: Ideal for streaming HD Movies, basic photo/video editing, casual gaming, handling large office documents and multitasking etc.

16 GB: Recommended for extensive multitasking operations, professional content creation software and high-end gaming usage.

The brain of your Laptop which determines the software you can run, operating speed and multitasking capabilities.

Intel processors can be found in all modern Macbooks and most Windows Laptops.

Core i9: One of the fastest processors designed specifically for people handling heavy tasks such as 3D animations, complex software programs and professional gaming.

Core i7: Preferred for users with higher performance demands like top-notch gaming, heavy multimedia usage (4K editing) and for working on multiple virtual machines.
Core i7
Core i5: Offers ideal performance for tasks like basic video editing, gaming, etc. It's more than enough for simple operations such as documentation, web surfing, etc.
Core i5
Core i3: Known as Intel’s budget processor which is preferred for basic tasks such as word processing, email, web surfing, watching videos, etc.
Core i3
Core M: This processor is designed specially for ultra slim laptops and provides extended battery life, low heat production and are found in fan-less design platforms.
Core M
Pentium, Celeron and Atom: These processors are found in the smaller or mini laptops, and are suitable for basic browsing and other everyday computing activities.
Pentium, Celeron & Atom
AMD: The best bet for students, budget gamers and individuals with straight-forward computing needs. AMD processors are a great choice as they are powerful and fast for most users while being comparatively cheap as well.
AMD

A longer battery life is very useful specially if you regularly use your Laptop on the go. A Laptop with 8 hours or more of battery life is ideal for people who want to go through most of the day without a charge.

Laptops come with integrated GPUs which are sufficient for most people since they cater to the basic computing activities, however, for people like hardcore gamers, video editors or graphic designers, a dedicated GPU would be required to support such tasks effortlessly.