Jar/Jad

I hope someone has an answer....if there is a jar and a jad file for a game or app. do they both need to be installed? If so, how? I have downloaded alot of free games and apps. but most have app. errors and wont run...whats up? Thanx....

First of all tell which mobile u using? if u r handset supports jar files then dont need jar files to be install. just install jar files with mobi mb or pc suit. & the problem of application errors is caused by unsupported or high resolution application. check ur handset manual for supporting