You can spend a lot of time researching water treatment and end up more confused about what you need than when you started.  Here we've tried to filter out the extraneous information so choosing the best system is a little more clear.  (Sorry... couldn't resist the puns.  If you pore over the research you'll see it's clogged with potential.)

The nasties

First of all, here's a list of who might be contaminating your water:

Protozoa: these single-celled organisms may look innocent but can wreak havoc on your intestinal system.  Giardia is a well known protozoan also known as "beaver fever".  Cryptosporidium is another one and is difficult to treat for because the cyst has a highly protective outer shell.  It can take from a couple of days to a few weeks for symptoms to manifest themselves after ingestion of a grumpy protozoa. 

Viruses: tiny but fierce!  They can slip through filters that have pore sizes bigger than .02 microns.  Some viruses to avoid include Norwalk, Hepatitis A, and Rotovirus.  

Bacteria: these guys are bigger and therefore the easiest to treat for.  E coli and dysentary are some well known bacterial baddies.

Ways to treat your water

Filters: Simply put, these push water through a filter commonly made of ceramic, hollow fiber tubes, or silica particles.  Traditionally you pump the water manually through the filter, but there are now gravity filters which require a lot less work.  There are also straw filters which pull the water through the filter as you suck it up through the straw.  Filters will get rid of bacteria and protozoa, including cysts, but unless the pore size is .02 microns or less they will not remove viruses.

Purifiers: Usually in the form of chemicals, purifiers attack the nasties in the water.  Iodine and chlorine dioxide are common purifiers and are sold in liquid and tablet form.  Ultraviolet light can also purify water by disassembling the DNA of the goobies in it so they can't affect you.  This method has been used in municipal water treatment systems for decades.   Purifying your water with chemicals or UV light will get rid of all the nasty organisms in it, including viruses.

Choosing a system

There are a lot of things to consider when choosing your system.  Most importantly is whether you need to treat for viruses or not.  If you stay in North America a filter is generally all you'll need, but if traveling overseas you will want something for viruses.  If you want the cleanest water possible filter it first, then purify it with chemicals or UV light. 

Other things to think about: will you be treating water for a group, or just yourself?  Is weight an issue?  Are you ok to wait for chemicals to work or do you need your water right away?  Will there be a lot of particulate in your water that might inhibit purification and require a filtration step?

Our recommendations

Here's a list of water treatment systems that we usually stock in the store, most of which we have personally used.  We've listed the pros and cons to help you pick what's right for your needs.

Gravity filters

If you only get one treatment system, these are a great choice.  Gravity does the work for you so you don't have to pump.  They filter multiple litres of water in only a few minutes so are great for groups.  They are heavier and bulkier than tablets or liquid chemicals but are on par with other filter systems.

Platypus Gravity Works  - A great system that we've used many times, the Gravity Works has two bags (a "dirty" and a "clean") so you can store your clean water without having to dispense it directly into your containers.  It's fast and simple to use and filters out protozoa and bacteria.

RapidPure Explorer Camp  - This filter uses elecotrastic adhesion to attract everything in the water - including viruses - to its surface, where they stick.  It is infused with anti-microbial silver and carbon to prevent bacterial growth in the filter.  This filter has passed all the tests for protozoa, bacteria and virus removal.  Because the filter doesn't depend on tiny pore sizes it works very quickly under very little pressure.

Pump filters 

These have been around a long time and are proven effective at removing protozoa and bacteria.  They also remove particulate and bad taste (especially if they have a carbon component); however they are heavier than chemicals and require you to actively pump the water through the filter, which can be tedious.

MSR Sweetwater  - One of the lightest filters on the market and easy to pump, the Sweetwater has an activated carbon core for removing nasty tastes.  The handle folds down so the entire unit fits into a pot set or can be easily tucked away in your gear.  If the filter slows down and gets clogged, it's easy to clean in the field without tools.  It isn't good for groups due to the pump rate.  This model has been out for awhile and has been very positively reviewed online.

Chemical tablets or liquids

The two common chemicals used for water treatment are iodine and chlorine.  There are three things that really make chemicals attractive: they are cheap, they are extremely compact and light weight, and they get rid of viruses as well as bacteria and protozoa.  The downsides are that it's not a good idea to ingest chemicals over longer periods of time, they can add a swimming pool flavour to your water, and they require a wait time before your water is safe to drink.  They also do nothing to remove particles or tastes out of your water.

Aquatabs - Each tablet dissolves into free available chlorine when added to water.  This doesn't colour or flavour your water, unlike iodine or chlorine dioxide.  They are safe and easy to use and have been used globally for about 15 years.  After dissolving in your water you need to wait 30 minutes (room temperature) before you can drink it.  If the water is dirty, filter it first (putting it through a coffee filter or t-shirt works in a pinch).

Pristine - This chlorine dioxide liquid comes in two bottles.  You mix the liquids in a measured dose and wait 5 minutes until it turns bright yellow, then add to your water and wait about 15 minutes for the chemical to work.  You may notice a subtle tinny flavour to the water.

Ultraviolet light

Using UV light to purify your water kills everything in it (viruses included) and renders it drinkable within a few minutes.  There are no wait times, no chemicals, no added flavours, and no pumping.  The cons are that these units are battery powered, so you have the limitations associated with that (weight, battery life, etc).  They also don't filter your water (unless you get a pre-filter) so debris and bad tastes remain.  These purifiers also do only a litre of water at a time so are better for personal use than groups.

SteriPen Classic or Ultra - Many of us have one of these and are really happy with it.  All you do is press a button and stir the bulb around in your water until the light indicates it is safe.  The bulb lasts up to 8000 activations and SteriPen will replace it after that if you register your product.  The difference between these two models is the Ultra is lighter than the Classic and has a USB rechargeable internal battery, but the Classic comes with a pre-filter.  This is also available separately.

Conclusion

As we already stated, it's generally accepted that if you stay in North America you probably only need to worry about bacteria and protozoa, which means pretty much any filter will do the job for you.  If you travel overseas be sure to get something that targets viruses.  Apart from that, it's merely a matter of choosing the features that are most important to you and narrowing things down from there.