I have created couple of graphs in R

d5 <- scan('d500.txt')
p1 <- acf(d5,lag.max = 36 ,type = c("covariance"),plot = TRUE)

Then the next one is correlation

p1 <- acf(d5,lag.max = 36 ,type = c("correlation"),plot = TRUE)

And pacf

p1 <- pacf(d5,lag.max = 36 ,plot = TRUE)

If I take a one difference operation

d51 <- diff(d5)
p1 <- acf(d51,lag.max = 36 ,type = c("correlation"),plot = TRUE)

And then again acf

and pacf

How to interpret this?What does sinusoidal acf(cov) shows?