How precisely are television advertising slots sold?

Silly questions I never thought to ask before (thank you, ABC, for
stuffing so many ads into "Lost" that my mind wandered off in these
directions...):

When an advertiser buys, say, a sixty-second spot on a show, who
decided when _during_ that show the commercial will be shown?
(That is, e.g., during the first ad break following the teaser, or
in the third of five mid-show breaks, or just before the closing
credits accompanied by scenes from next week.)

Is that specified in the agreement/contract between the advertiser
and the network -- for the sake of simplicity, let's limit the
discussion to networks, and leave local stations out of it -- or is
it wholly up to the network? And if the advertiser does get some
choice as to when during the show the ad will run, do networks ever
charge different prices for different times?

Similarly, it occurs to me that the position of an ad during a
commercial break might affect its value: the first ad that runs in,
say, a four-minute break catches the viewers while they're still
paying some degree of attention; by the third or fourth minute they
may be in "Is the show back on? If 'no,' then ignore lights and
sounds emanating from tv" mode. How is it decided when an ad will
run within a commercial break?

Anim8rFSK...

I haven't been involved in the media buy in years, but you used to be
able to buy a show, or the first or last half hour, or just a night or a
week or whatever, bumpable or not, depending on what you want to spend.

I do a lot of work for an agency that buys 'play them when you want'
slots, and the stations put them all together, so you'll see three or
four of my ads at once in little mini marathons in the middle of the
night :-)