An offer letter normally confirms the job title, salary and benefits, working hours and location, start date, any probation period and the conditions the offer depends on, such as satisfactory references or a right to work check. In the UK, it is often followed by a written statement of employment particulars or a full contract, which employees are entitled to from their first day. The letter usually asks the candidate to accept in writing by a set date.
The offer is the point where a candidate's view of the whole process settles, so a slow or confusing letter can undo good work earlier on. Sending it promptly after a verbal offer, keeping the terms consistent with what was discussed and stating conditions clearly all reduce last-minute declines. Requirements differ by country, so letters are usually checked against local employment rules.