It's a tough tech life to try to keep using your old stuff

Here's how that happens: You have some apps that you were using on an older OS X. Now, you have a newer Mac, or you have upgraded to a newer version of OS X that no longer supports PowerPC apps.
Your new Mac and OS X system won't support the apps that you want to use, but there's several workarounds for that.
If your Mac can still boot to the older OS X system, then that's one choice: Keep your games or other apps with the older OS X system, maybe on an external drive. Boot to that older system when you want to play (or work

) with the older apps.
It's also possible to run an older OS X system in a virtual machine (using Parallels or VMWare, etc). I don't know how to set that up, but there's others on this site that can give some info about do ing that.
Or, you could keep an older Mac around just for your chosen games and apps.
Or - you could actually give us the game or app names - there may be other possibilities that you haven't considered yet.