[ WHAT IS NINJUTSU ]

Honestly, the best way to describe it is as anything that isn't taijutsu or genjutsu. This is because ninjutsu is a very wide scope, consisting of techniques from firestyle to even weaponry.

Think of ninjutsu as a physical manifestation of one's chakra, meaning that ninjutsu is a jutsu that uses chakra to affect the opponent physically (as opposed to genjutsu, but we'll get to that in another chapter)

Most ninjutsu use shape transformation, which deals with controlling the form and movement of the ninjutsu, while nature transformation deals with changing the ninjutsu's physical properties. Ninjutsu that use nature transformation tend to follow a naming convention of their nature type followed by a more specific name. With practice and experience, certain ninjutsu can become second nature to a ninja, allowing them to perform it at will. (Narutopedia)

[ SHAPE TRANSFORMATION ]

Shape transformation is an advanced form of chakra control, that involves manipulating one's chakra's form and movement.

The rasengan is the highest form of shape transformation, bc the rasengan is complete chakra. Puppet threads also are shape transformation

[ NATURE TRANSFORMATION ]
According to narutopedia : Nature Transformation (性質変化, Seishitsu Henka, English TV: Change in Chakra Nature) is an advanced form of chakra control that entails the moulding and defining of the nature of one's chakra, altering its properties and characteristics for use in techniques. 

There are five basic types of nature transformation (also known as elementals) called EARTH, WATER, WIND, FIRE AND LIGHTNING.

>>Fire is strong against Wind but weak against Water.

>>Wind (風, Kaze) is strong against Lightning but weak against Fire.

>>Lightning (雷, Kaminari) is strong against Earth but weak against Wind.

>>Earth (土, Tsuchi) is strong against Water but weak against Lightning.

>>Water (水, Mizu) is strong against Fire but weak against Earth.

Narutopedia : Basically, if an elemental technique is put against another elemental technique of the same level, but of a stronger nature, then the technique with the superior nature will prevail. However, a technique with a weaker nature can overpower a technique with a stronger nature if the former is of a higher level. For example, fire can overpower a water technique if it is first strengthened by a wind technique.

A similar principle applies in how technique of the same element interact, what is called counterbalancing. If two techniques of the same nature and the same level are used against each other, they will cancel each other out, as when Lightning Release: False Darkness is deflected with Lightning Cutter. However, if the two techniques have different levels, the higher-level technique will not only prevail but will grow stronger from interaction with the weaker one

[ AFFINITY]

Affinity is a person's natural inclination towards an elemental. It can sometimes be genetic, like with the Uchiha's Fire affinity (in most cases).

Narutopedia : One's affinity can be determined using pieces of paper made from a special type of tree that is grown and fed with chakra. When this so-called "Chakra Induction Paper" (チャクラ感応紙, Chakura Kannōshi, English TV: Litmus Paper) is exposed to even the slightest hint of chakra, it reacts according to the chakra's latent element:

Fire: the paper will ignite and turn to ash.

Wind: the paper will split in two.

Lightning: the paper will wrinkle.

Earth: the paper will turn to dirt and crumble away.

Water: the paper will become wet/damp.

Now just because someone has an affinity towards an elemental, doesn't mean they can't learn another one. Although it takes a lot of  training to master even their own affinity, so it will be hard to master another.

Hashirama, Tobirama, Orichimaru, Sarutobi and Mu are the only shinobi known to have mastered the 5 elementals through training.

Other ways to master all include :

Possession of the Rinnegan to shorten the learning process.

Acquiring the Six Paths Senjutsu and Truth-Seeking Balls.

Possession of the Bashōsen allows any shinobi the usage of all five natures.

Using Earth Grudge Fear to gather hearts with other chakra natures.

In the anime, Fūka got them by stealing others' chakra, though due to the accumulated amounts surpassing one body's limits, she had to divide them in various combinations in multiple bodies.

In Naruto Shippūden the Movie: The Will of Fire, it was stated one who absorbs five kekkei genkai with the Chimera Technique would gain a mastery over all five natures.

In Naruto Shippūden: Dragon Blade Chronicles, Naruto gets the Dragon Blade, which allows him to use all five natures

Meh I'm too lazy to describe the details of each elemental rn so just take this first imma get to it in another chapter woooo.