The tantric scriptures of shaivism are the shivAgamas. These along with the vedas are considered the holy scriptures of the religion. Both vedas and Agamas are blessed to us by the God shiva Itself, through the divines and sages. Like the word veda indicate knowledge the root gam in the word Agama indicate the dynamism. (Agama is interpreted as the one that "arrived" from God). As the name indicate Agamas are the dynamism of the supreme knowledge. It is the implementation or recipe for attaining the Supreme Truth. If vedas are the science, Agamas are the engineering.
Agamas elucidate four glorious parts/paths called padas. They are
There are 28 shivAgamas. All of them have these four parts. The order and the name change. (For e.g. in some of the Agamas it could be called vidya pAda etc.) These four are the worship procedures prescribed for the devotees.
Agamas, like vedas are the sources of almost all the philosophical doctrines and the religious life of Hinduism in general and shaivism in specific. It enunciates the nature of the Supreme as well as the way to get to It. Agamas advocate the practical aspects (implementation aspects) of religious life and regulations that would make the worship streamlined to produce the ultimate benefit. So these have quite good elaboration of the rituals. These glorious rituals could be the public worship performed at temples or the personal ones performed at the residence. The rules of Agamas can be followed by any person without bothering about caste, color, creed or sex. The core philosophy the Agama tantras explain are about the three things pati, pashu, pAsha . Some of the Agamas appear to put forth non-dualistic and some others the concept of more than one "real" thing.
vedas are called nigama and along with Agama, they are regarded as shR^iti. vedas are full of mantras and so are in the form of aphorisms, but the Agamas are in the form of discussion and so are available in the simple verse structure. Both vedas and Agamas were revealed to the world by the riShis from their spiritual vision.
The Agamas originated from the Supreme shiva's five faces, namely sadyojAta, vAmadeva, aghora, tatpurusha, Ishana. There are two classifications of Agamas. They areshivAgamAs and rudrAgamas depending upon who the first propogator of the corresponding Agama were (shivas or rudras). There are 10 shivAgamas and 18 rudrAgamas. These Agamas form the basis of the worship of Lord shiva.
These Agamas also have upAgamas - the supporting Agama tantra texts. These are 204 in count for the 28 Agamas. Some of the Agamas are said to be having trillions of verses. For example kAmika, sUkshma and sahasra Agamas. Lots of these texts have been lost out in the history. Many of the Agamas available today are not having all the parts in full. Some of the Agamas have only some of the pAdas available.