I have a question about playing different versions of Minecraft.

If I were to download Minecraft right now, I'm assuming I would get v. 1.2.5. When 1.3 comes out in August, I guess I would update(?). After I have 1.3 can I still play v. 1.2.5 if I want or can you only play the latest version once you have it? Also, once I buy Minecraft, can I play other previous versions. My guess would be that you can play all the versions, because I know people also play betas and alphas. I was just wondering. Thanks.