Janta is doubling a note with stress on the second note obtained by momentarily touching a lower note (crushed note) as in the phrase sa-sa-ri-ri-ga-ga-ma-ma or Sa-sa-ni-ni-da-da-pa-pa in Maayaamaalavagowla. In ascent Janta is also called Sphuritham and in descent Prathyaaghaatham. The lower note need not be present in that Raagam but the next lower note in the 12 note scheme or an intermediate point. The second note may also be 'akaara' i.e. without strumming(plucked instruments) or continuous blowing (winds). Listen to akaara Janta (Aarohanam) and akaara Janta (Avarohanam). Janta imparts 'Ghanam' to the music. when the note is repeated in descent an upper note's pitch may be heard momentarily (especially on the Veena).
There are others like 'Khandippu' (touching an intermediate note quickly in a descent), 'Orikkai' (wavy descent), 'Ravai' etc. which are not explained here. The explanations by Subbarama Deekshithar along with symbols used by him for different Gamakams are available at http://www.ibiblio.org/guruguha/ssp_cakram1-4.pdf(page 24). All the notations in Sangeetha Sampradayaprarasini have the symbols.
While Gamakam imparts beauty and identity to the music, it also makes it more difficult to identify the note, especially when the oscillation is over a large range. The same movement may be assigned different notation. For instance the oscillation from da2 to Sa (or a slightly lower point) will be 'da' in the phrase 'pa da ni Sa' in Kalyaani (which uses ni3), but it will be 'ni' in the phrase 'pa da ni Sa' in Bhairavi (which uses ni2 and the da2 is straight). But by association with known phrases of the same Raagam, the notation as a phrase could be identified. If you develop this ability you can identiry less common Raagams, if you can identify the Aadhara Shadjam, the range sa to pa and the range pa to Sa. Here is an example - Aarohanam of Sooryakantham has the Poorvaangam of Maayaamaalavagowla and Uttharaangam of Sankaraabharanam. Also listen to full Arohanam of Sankaraabharanam and full Aarohanam of Maayaamaalavagowla.
Though the nature of Gamakam (or the absence of it) on a note depends on the Raagam, some generalisations can be made. Notes ri2 and da2 are held without gamakam if followed by ga2 or ni2; if not they may have Gamakam or be played straight. Note ga3 (Anthara Gaandhaaram) is a srong note, often held straight or given slight Kampitham. Notes ma2 and ni3 are generally held in conjunction with pa and Sa respectively. Note ma1 is held plain if ga3 is not there. Notes ga2 and ni2 generally have Gamakam and the range depends upon the existance of ri2 or da2 in that Raagam. There are special Gamakams in Raagams like Begada, Sahana etc.