Your friend is correct. There ARE many types of strength. Let's take a quick look at each...
Limit Strength: this is defined as the maximum amount of force your muscles can produce. This type of strength is not likely to be seen except under the most extreme of circumstances, such as life-threatening situations or under the influence of certain drugs...PCP for example.
Maximal Strength: this is defined a the most force your muscles can produce voluntarily. Maximal strength includes concentric, isometric, and eccentric muscular contractions. The human body can voluntarily produce the greatest force during an eccentric (negative) contraction.
Absolute Strength: this is defined as the maximum force an athlete can generate, irrespective of bodyweight and time of force development. Absolute strength is important to lineman in the sport of football, for example.
Relative Strength: this is defined as the maximum force an athlete can generate per unit of bodyweight, irrespective of time of force development. This type of strength is important to jumpers and gymnasts, as well as wrestlers and boxers.
Optimal Strength: this is defined as the ideal level of strength needed so that any further development would not improve perfromance. A distance runner only needs to have a certain amount of upper body strength to maintain proper running form. A 400 lb bench press is not necessary for him.
Strength Endurance: this is defined as the capacity of a muscle to maintain consistent force output with repeated contractions over time at a percentage of maximal strength superior to 30%. This type of strength is important for sports such as swimming and cross-country skiing.
Speed Strength: this is defined as the ability of the neuromuscular system to produce the greatest possible force in the shortest possible time frame. Speed strength is further broken down into 3 types...
1- Starting Strength: the ability to produce a high level of force at the beginning of a muscular contraction.
2- Explosive Strength: the ability of the neuromuscular system to continue developing the already initiated force as quickly as possible.
3- Reactive Strength: the ability to switch rapidly from an eccentric to a concentric contraction.
So as you can see, the word strength is far more complex than you might have thought. Perhaps now you have a little better understanding of their differences and uses in the athletic world.