is my internet activities known to my isp?

unless you use a proxy server, then yes

They do keep logs. I hope you aren't doing anything illegal.

Abolutely yes, unless you use a proxy.
Any really interesting activity by you on suspisious child porn, terrorist and other nasty sites you use are filtered and examined by the FBI. Im not sure that even proxy sites are safe from that (I may be incorrect but there is some evidence that that is tracked by the big boys as well.
So you better be nice or it wont be SAnta calling!