What does C5 and S7 stand for?

That are classifications in disabled sports, showing the level of disability. ('S' = swimming, 'C' = cycling)

Swimmers are classified according to how their impairment affects their ability to perform each stroke.

The classification rules of the International Federation for Swimming state that athletes with a physical impairment, visual impairment and intellectual impairment are eligible to compete in this sport at the Paralympics.

Classification also groups athletes in classes, defined by the degree to which they are limited in their ability to perform each stroke. Because of the number of different impairment types that compete in swimming combined with the fact that freestyle, butterfly, backstroke and breaststroke each place different demands on the swimmer, means swimming has a high number of classes.

There are several different classes in Swimming:

1–10: athletes with physical impairments. Class 1 swimmers’ impairment has the greatest impact on their ability to perform strokes; class 10 swimmers’ has the least impact.
11–13: athletes with a visual impairment. Class 11 swimmers have little or no sight; class 13 swimmers have limited sight.
14: athletes with an intellectual impairment compete in class 14.

Breaststroke uses greater leg propulsion than any other stroke, therefore athletes with a physical impairment often have a different class for this event compared to Freestyle, Backstroke and Butterfly. This is also taken into account when athletes compete in the Individual Medley. This is shown by the following prefix:

S before the class represents Freestyle, Backstroke and Butterfly events.
SB before the class represents Breaststroke events.
SM before the class represents Individual Medley events.

So for example an SB5 swimmer will have an impairment that will have a greater impact in the water while swimming breaststroke than an SB7 swimmer. Also that same SB5 swimmer may be an S4, S5, S6 or S7 swimmer for freestyle due to the impact their impairment has on their leg kick in the water.

http://www.london2012.com/paralympics/swimming/classification

The classification rules of the International Federation for Cycling state that athletes with a physical impairment and athletes with a visual impairment are eligible to compete in the sport at the Paralympics.

Classification also groups athletes in classes, defined by the degree to which they are limited in their ability to perform activities within that sport. In Cycling these classes also define which type of cycle you compete on: bicycle, tricyle, handcycle or tandem.

There are four classes for competition:

B – athletes with a visual impairment who compete on a tandem with a sighted pilot on the front
H1–H4 – athletes with an impairment that affects their legs and so compete using a handcycle
T1–T2 – athletes with an impairment that affects their balance and so compete using a tricycle
C1–C5 – athletes with an impairment that affects their legs, arms and/or trunk but compete using a standard bicycle

In the H, T and C classes, the lower the athlete’s class number, the greater the impact of their impairment on their ability to cycle. So for example a C1 cyclist will have an impairment that has more of an impact on their ability to cycle than a C5 cyclist.

http://www.london2012.com/paralympics/cycling-track/classification
http://www.london2012.com/paralympics/cycling-road/classification