I have to install the rjson package in R and looking at the CRAN page that deals with the package I saw that rjson has different dependencies:
Reverse depends: couchDB, df2json, edeR, gooJSON, indicoio, kintone, notifyR, RDSTK, Rfacebook, rJython, Rlabkey, rPlant, RYoudaoTranslate, SmarterPoland, sotkanet, source.gist, Storm, streamR, tibbrConnector, zendeskR;
Reverse imports: AntWeb, argparse, BerlinData, blsAPI, Causata, d3Network, db.r, geonames, GetoptLong, ggmap, h2o, helsinki, james.analysis, meteoForecast, mpoly, networkD3, ngramr, nhlscrapr, OpasnetUtils, OutbreakTools, paleobioDB, RAdwords, rbefdata, rClinicalCodes, rfisheries, rglobi, RIGHT, rnrfa, solr, StereoMorph, structSSI, twitteR, vdmR, yhatr;
Reverse suggests: fuzzyMM, GSIF, installr, mlr, plotKML, rsnps, sorvi, trajectories;
What is the difference among them and do I have to install all of them to use the rjson package?
These are reverse dependencies, that is these packages depend on rjson.
You do not have to install these in order to use rjson.
Looking at the DESCRIPTION file (this is where the dependencies are stated) you see only:
Depends: R (>= 3.1.0)
So rjson package needs only R newer or equal than 3.1.0 to run.