All Pāli roots known to us are collected here from various sources, including Pāli Dhātupāṭha (pāṭ), Dhātumañjūsā (mañ), Saddanīti Dhātumālā (māl), and Dhātvatthasaṅgaha (saṅ). Hence, this is the most exhaustive collection of Pāli roots.
The data of Pāli Dhātupāṭha are taken from the book edited by Dines Andersen and Helmer Smith (1921). Those in Dhātumañjūsā are taken from CSCD. If any doubt occurs, please consult the book directly. There are things to be concerned about the two. First, as for Dhātupāṭha is used by Moggallāna school whereas Dhātumañjūsā is used by Kaccayāna school, the two books use slightly different naming and grouping scheme. The former has 9 groups, the latter 7 (according to Andersen and Smith's book). However, schools following Kaccāyana scheme widely use 8-group classification, including Saddanīti and Dhātvatthasaṅgaha.
To prevent confusion, I therefore treat the issue this way. I use lowercase Roman numbers for the 9-group scheme, i.e., i-ix, and uppercase for the 8-group, i.e., I-VIII. To make the system consistent, so I put Dhātumañjūsā in the 8-group by changing group VI and VII described by Andersen and Smith to group VII tanu and VIII cura respectively. This means Dhātumañjūsā does not have group VI gaha. The word gaha itself belongs to group V kī in this book.
Second, concerning the numbers, Dhātupāṭha gives the total number of roots as 639, but some roots have a variation, which repeats the count. So, we see 643 totally in the finder (some numbers are repeated, i.e., 547, 554, 563, and 609). In Dhātumañjūsā we have 885 roots totally (Andersen and Smith miscounted one root in number 526-7), and stanza numbers are used as references (not conforming to the Andersen and Smith's book).
Third, I spent several days checking root names and definitions in Dhātumañjūsā against Thai edition (Wat Chakdaeng, 2013). So, we can see a number of variants in this book (if the user selects the option). The variants are marked by sy (Syāma). It is worth paying attention to the Thai variants because they underwent some validation and meaning checking (but several roots are still unknown to Thai experts). The variations found in the Andersen and Smith's book are marked likewise with as.
Finally, unlike Dhātupāṭha that most definitions are kept intact (except double-character fixes), I edit definitions in Dhātumañjūsā by removing unrelated words (e.g., ca and other fillers) to make them less distracting. By the constraints of prosodic meter, reading directly from the source may cause new learners a hard time. So, here all roots and definitions are made clear by their form. Still, some cryptic and circular definitions can be found.
Saddanīti Dhātumālā also has variants. They are marked by sy (Thai, Bhūmibalo edition, 2016), and sm (the Smith's edition). It is worth noting that several roots in Dhātumālā, 1687 distinctively counted by Smith, are duplicate here, and the numbers given are definition numbers not the counting.
The recent Dhātvatthasaṅgaha was composed by the Thera of Visuddhārāma in Mandalay. This book can fulfill what are missing in the old texts. Several days are spent to make the data programmable. And several corrections are made against Thai edition of Dhātvatthasaṅgahapāṭhanissaya (MCU Press, 1992), in which some noteworthy variants are found (marked by sy). The reference numbers used in this book are stanzas' number, which can link to the text directly. I do not have enough time to recheck the whole thing. If anyone finds unusual instances, please kindly inform me.
Typically, definitions of roots are given in locative case. It sounds like "in (the meaning of) ..." For example, gatiyaṃ means in the meaning of going. We can translate it simply as going. To find English meanings of common definitions, see Meanings of Pāli Roots.
The program here is quite powerful. It can give you perspectives unaware to ancient scholars. Features in the finder are self-explained, so no detailed guidance is needed. The user just has to play around and experiment. However, you should know that filtering operates before grouping by Unique function. If you want to search in the Unique result, it is better to use search facility in the browser.