The terms seṭ and aniṭ refer to classes of roots in Sanskrit grammar. In the terminology of Pāṇini, seṭ (from sa-iṭ, Aṣṭādhyāyī 1.2.18, 6.4.121) means "with an i-sound", and an-iṭ (Aṣṭādhyāyī 3.1.45, 6.1.188, 6.4.51, 7.2.61) means "without an i-sound".
The i sound in question is a phoneme i that appears in certain morphological circumstances for certain, lexically defined roots, regularly continuing Proto-Indo-European (PIE) laryngeals, as in *bʰéuH-tu-m > bhav-i-tum. Note that the PIE laryngeal (represented by an *H here) was a part of the PIE root; it occurs in all of its allomorphs, for example *bʰuH-tó-s > bhū-ta-ḥ (*bʰeuH- is reduced to *bʰuH- in PIE due to ablaut; the laryngeal disappears in this context, leaving its trace in the length of ū in Sanskrit). In Classical Sanskrit, the scope of this i was broadened by analogous change.
In Aṣṭādhyāyī the synchronic analysis of the phenomenon is somewhat different: the i sound is treated as an augment (called āgama in the terminology of the later Paniniya school) of the suffix that follows the root. (The ṭ marker, defined in 1.1.46, indicates that the i should be appended at the beginning of the form, viz. the suffix.) For example the rule 7.2.35 states that i should be prepended to ārdhadhātuka suffixes beginning with a consonant other than y; an example of such suffix is -tum (the Classical Sanskrit infinitive); this rule is restricted by the following sūtras. Note that this analysis, being synchronic, does not contradict the diachronic analysis of PIE origin of the phenomenon.
An example of differences between the two classes is the aorist-marker. While some of the aniṭ-roots form aorist with the -s suffix, seṭ-roots are suffixed by -iṣ.
Following this terminology, PIE roots ending in laryngeals are also called seṭ-roots, and all others aniṭ-roots.