Regular expression notation clarification

For the alphabet $ Σ$ ={a,b,c}

I was wondering how you would say:

T that has elements from Σ, so could be T=a, T=bc

I was considering maybe $ Σ^*$ or $ Σ^+$ would describe that, but I am not sure what both of them mean.

I guess $ Σ^*$ ={aa, aaa, ba, abc, …} and not sure what $ Σ^+$ is.