What kind of bugs are these?

While I was looking about my lemon tree, I saw a small branch that had these green 'bumps' where ants were hovering about. What are these green 'bumps'? Is it good or bad? If bad, how do I get rid of them or prevent any further spreading?

Thank you.

 

That looks like scale to me. Oil spray-- spray late evening or early in the morning, never when temp is over 85 or in bright sun.

 

Looks like scale being farmed by ants. Treat with insecticidal soap or horticultural oil.

 

Yup, definitely scale. The ants eat the "honeydew" excreted by the little buggers