LATCH "Lower Anchors" are the attachment points between the seatback and seat bottom. They're meant to attach to a strap or rod from the bottom of the chid restraint.
"Top Tether Anchors" are the attachment points behind the seats, to attach to a strap connected to the top of the car seat.
Almost all forward-facing car seat benefit from or absolutely require that you attach the top tether anchor strap to one of the anchorage points. The second row seat has three of them.
Not all car seats are LATCH compatible (attachment at the bottom of the seat); they may only work with seatbelts to secure the seat.
There are relatively few rear-facing seats (infants under 20 lbs
and not yet a year old) that are LATCH-compatible. Graco does make a LATCH retrofit kit that will work with some of its SnugRide infant seats (but not all).
LATCH is becoming much more common for convertible (rear-facing and forward-facing seats) seats, as well as forward-facing only seats. Most of those seats can still work with belts.
Britax is releasing a new line of seats that are LATCH-or-belt compatible. They include an infant seat (that has a detachable base and goes to 30 lbs!), a LATCH version of their ubiquitous Roundabout convertible seat, and other convertible seats (including one that goes to about 65 lbs). They're expensive but a lot of people think they're welll worth it.
Generally, installing a car seat in the center position is the safest. This keeps it away from side impacts. I'm not sure if a LATCH seat can straddle the two outboard LATCH pairs, it would depend on the seat design and width between the two inner LATCH points (and probably prevent another LATCH seat from being used).
Some folks are nervous about installing a rear-facing seat in the center position because they fear the armrest coming down and hitting the infant. That's a decision each parent should make individually, based on their vehicle, their seat, and their comfort level. Many infant seats have a lip so high that a flapping-down armrest simply can't contact the infant. And, side impacts favor the center position more than the outboard positions.
That aforementioned upcoming Britax infant seat has a solution -- a unique "stabilizer bar" that is designed to keep the rear-facing seat from a rebound toward the back. I'd imagine that bar would also keep an armrest in the tucked position too.
http://www.childseat.org/whats_new/wne_detail.cfm?wne_id=29