Usually, to seep is to go slowly through a porous material. (example - Blood seeped through the bandage.)
Usually, to leak is to go through a hole or a crack. (example - Water leaked from the cracked pipe.)
New Oxford American Dictionary
seep | sip |
verb
flow or leak slowly through porous material or small holes
leak | lik |
verb
1 accidentally lose or admit contents, especially liquid or gas, through a hole or crack