Someone explain the different Defeneses we use

[gorki11]

I understand the 4-2-5. Is I know the two represents the two box LBs. But in the 4-2-5 set is that where we use the star/hybrid? People will also say we are in our base D. I always thought a base D was the standard 4-3. And then their are other times where I guess we play a lot of nickel which is 4-2-5, but instead of the star/hybrid they just bring an extra DB in. Am I correct about this if not, someone please correct me.