Well, basically the MR games are raisng sims in which you raise your monster that you produced from whats called the "shrine" by using CD/DVDs(MR1-4) or by passwords(MR advanced, advanced2), competing them in tournaments or bringing them on expeditions to remote sites.
Basically you raise it like you would a virtual pet from the day of its birth till the day of its death(ya, sadly they die by age, constant K-O's or by abuse, except in Monster Rancher 4, MR Advanced and Advanced 2 which they wont let you use the monster anymore if it gets too old). You can pit it in combat which is nothing like Pokemon or something. Its actually very creative.
I recomend starting out w/MR advanced games or MR3, unless you know someone who is selling their MR1 or 2 because there are very few copies left in the world of those.
for more info head on to
http://www.monsterranchermetropolis.com and for pics of monsters visit
http://www.legendcup.comP.S like I said before, in MR 1-4 you put in any disc and it will give you a monster and I mean every disc. I personally only own the MR advanced games but tommorow my friend is gunna give me his MR2 and I just need 98 more dollars and thirty-seven cents to buy a PS2, MR3-4. I like the MR games because there is literally no end to this game and its just fun. You really get into it after a while. :)