As a general rule, carbon tips are stiffer and tend to have a higher strength rating, normally in the 2.5oz to 5oz range. These are best used on rivers carrying a bit of flow, but they can be quite brittle.
Glass tips are more forgiving and have a soft, progressive action They’re produced in lower test curves (0.5oz to a maximum of 2.5oz) and are better bet for shy-biting species on stillwaters, or slow-moving rivers.
Generally, carbon tips have a dark base, whereas glass tips have a clear or white base.
Here's when you should use each...
Tips