Are condoms effective for preventing std's?

To some degree: But remember some STDs are spread by any skin-to-infected-skin contact, and others by infected genital secretions. So, unless you wear a whole body condom, there will likely still be some contact with potentially infected skin and secretions and therefore some risk.

Yes but not perfect: Condoms are effective in preventing stds. However, they are not 100% perfect in preventing stds because they cannot cover all genital surfaces, but for the most part, they are effective.

Much of the time: There can be still be skin to skin contact with a condom- so some infections may still be spread.