It's a policy that covers against lawsuits that might be brought against you because of damage caused to third party properties, injury to spectators or innocent bystanders, etc.
The policy generally does not have a deductible for third party bodily injury but there is a deductible for third party property damage.
Yes, the above policy also covers defense costs. This could either be within the policy annual limits or over and above the policy annual limits depending on your requirements.
The amount of cover that your business will require largely depends on the level of risk that you undertake, nature of your works, your surrounding property exposures etc. You will probably find that there are accepted minimum standards of the particular industry you operate in, and there may be requirements specified by your clients. Before making a decision on how much cover to buy, you should always check out both of the above.