Noseki - 4-4, low approach, attach on top, peep
When black attaches on top with , white normally plays hane at . This page discusses what happens if white instead peeps with immediately.
Peeping first is not joseki, and is locally bad. Only under special circumstances is this move playable, see: Pro play example
The peep might seem attractive when white is afraid of at a (taking the corner) or b (a sacrifice to potentially overconcentrate white)
For black, the normal response to is . White can next try several moves, like the hane at a, the extension to b and c, or the jump to d
After the hane of , black can cut at , and sacrifice two stones. With , black gets influence in the center, after which black forces white to capture the two sacrifice stones with and (white captures at a).
Simply extending to is slow and unappealing. Black takes the corner with , and white is forced to make a base for his heavy group with . White is clearly worse of than in the normal joseki
If white tries to play lighter with , black can still block in the corner. White then makes shape with , but his position is cramped.
Black might also try the peep if influence is more important. Now, white will have to deal with the risk of black pushing through and cutting. is one option. White's position is low, and black can still take the corner with (which can also be elsewhere)
If white plays like this, black will hane. If white plays counter hane with at , black will get to play hane at the head of three on both sides. So white will probably extend, allowing black to take the corner.