A first aid kit is a collection of medical supplies in a container to enable any explorer to give basic first response medical care.
Carrying a basic first aid kit is a good idea for new explorers, those who like to be careful, or those visiting particularly risky sites. Some people regard it as an essential piece of equipment; others find carrying one an unnecessary weight.
A first aid kit needn't be bulky, heavy or expensive; a cost-effective idea is to build your own (keeping in mind that most store-bought kits are very basic for their price), which lets you customize it for your needs. For example, if you're concerned about starting out (say, slipping and getting badly cut in a drain), then a small bottle of antiseptic and dressings/bandages, all enclosed in a small watertight case (like a lunchbox) would be a good choice. This would allow you to either patch yourself up and keep on going (knowing that the wound won't get infected) or stop the bleeding and get back outside to treat it.
A few suggestions of possible UE-relevant contents are listed here; by no means do these *have* to be carried since contents (or lack thereof) are an entirely personal decision. All of these can be easily and cheaply bought through online auction sites or online pharmacies:
Non-latex gloves Used to safely and cleanly treat wounds. Another potential use is for removing drain slime/jelly cleanly (click here for example).
Antiseptic (liquid, cream or spray) Liquid antiseptics are best for large kits, where a dish or container to dilute the solution with clean water can be included. Sprays and creams are a good choice for smaller kits, though for deep cuts the wound will still need to be irrigated with clean water to clear out contaminants before the antiseptic is applied.
Flashlight (AAA) Useful for checking for a concussion. Could also be used as a "last resort" backup torch if your other flashlights malfunction.
Dressings Absorbent dressings and cotton gauze pads are good for stopping bleeding, though adhesives are fine on smaller cuts (and won't get mucked up in drains). Pack at least two 5cm x 5cm dressings, as well as two 7.5cm x 5cm (or nearest equivalent sizes). Larger sizes (such as 10cm x 10cm) are available, but difficult to pack into a compact kit.
Bandages Used for two purposes - to hold dressings in place (conforming) or for applying a lot of pressure (elastic). Having a couple of 5cm and 7.5cm wide bandages of both types is highly recommended. Conforming bandages are made in a variety of materials for applying different amounts of pressure; for a compact kit, buy the heaviest weave type that you're willing to pay for.
Thermal blanket Useful for treating shock and keeping a casualty comfortable. They're fairly compact and also cheap, so including one is a very good idea.
CPR facemask Provides a barrier between a first aider and a casualty to prevent the transmission of any contaminants when performing CPR.
Generally the items which are most likely to be used should be placed at the top "layer" of the box (for example, gloves and bandages), then followed by any less-used yet still necessary items (such as small dressings, burns dressings etc.).