"Fortis" and "lenis" are not terms which are currently not accepted in standard modern phonetics and phonology (e.g., they do not appear in the index or glossary of "A Course in Phonetics" by Peter Ladefoged, the standard reference handbook of phonetics). Googling "fortis lenis consonants" brings up:
(1) a Wikipedia entry which says the terms "refer to the opposition of consonants such as p, t vs. b, d. In a narrow sense, fortis refer to consonants such as p, t pronounced with tenseness (more muscular tension) and lenis to consonants such as b, d pronounced without." The part "tenseness (more muscular tension)" is complete nonsense-- the difference between p and b, and t and d, is the presence or absence of vocal fold vibration. The article becomes more inaccurate from here, but I won't go into detail.
https://en.wikipedia.org/wiki/Fortis_and_lenis
(2) A page from a book about English phonology which cites the same examples and is basically using them to refer to a voiced/unvoiced distinction. (
http://books.google.com/books?id=gGCZgBo4qSAC&pg=PA77&lpg=PA77&dq=fortis+lenis+consonant&source=web&ots=FjNVtwUdQK&sig=u2wXNDX4g24MEU3_TuAsRyhuUjg&hl=en&sa=X&oi=book_result&resnum=3&ct=result)
(3) A course website which uses the above, plus an example of Icelandic where the terms "fortis" and "lenis" are used to described a voiced/aspirated contrast in Icelandic.
https://notendur.hi.is/peturk/KENNSLA/02/TOP/fortlen.html
(4) Some articles about Zapotec which invoke "fortis" and "lenis" to describe two sets of consonants at the same point of articulation which only differ by a small set of features.
http://www.liquisearch.com/list_of_zapotec_languages/phonetics_and_phonology/fortis__lenis
In short, which features make a consonant "fortis" or "lenis" depends on how those terms are understood to be defined for the given language in question. Hard to proceed further if you don't say which language you're interested in.
