This is a link where the differences are explained.
Depending on how you look at it, this may be a reason to use CRM for Outlook or it may be a reason to use the Web Client. CRM for Outlook leverages some great Outlook functionality (like categorizing, flagging for follow-up or conditionally formatting your CRM lists). On the other hand, individuals that use this functionality tend to be Outlook power users. In addition, these settings do not update any CRM data (so, for example, if a user categorized a contact using Outlook categories, this information will not be stored in CRM, making it useful only to the single user who set the category). Plus, if you’ll click the image, you’ll notice that CRM for Outlook tends to make a lot of text look blurry (only joking, that’s just text that we were hiding in the image). The Web Client, on the other hand, has some excellent functionality that is not available in CRM for Outlook.
Many users keep Outlook open to their inbox and want to actively monitor their email throughout the day – they think of Outlook as mostly an email tool (and to a lesser extent a calendar and contact management tool). When users are forced to access CRM within Outlook, they must then constantly toggle between the various modules of CRM and their Outlook inbox. This can be somewhat mitigated by just opening the CRM for Outlook area in a separate window – but few users seem to remember to take this action. Many users like the idea of have two applications that they can “ALT+TAB” between rather than having to constantly use the mouse to navigate between different parts of Outlook to do their jobs.
Most business leaders assume that, because CRM can work with Outlook, it will be easier to learn. While this may be true for “Outlook Power Users”, it is not necessarily the case with everyday users. When you decide to train users on CRM for Outlook, it creates a need to explain how CRM integrates with Outlook in some detail. Users will need to grasp how CRM integrates with existing Outlook items (i.e. email, contacts, appointments) and how CRM also has it’s own items (i.e. leads, accounts, opportunities) that are also available within the Outlook interface. What seems to create more confusion is that users now have two places to go, inside of Outlook, for what should appear to be the same information (i.e. CRM has contacts and Outlook has contacts; CRM has appointments and Outlook has appointments). Because CRM and Outlook have different sets of records to track the same information this can be confusing for new users. By starting with the Web Client, users can be eased into basic CRM concepts without having to learn the details of how data is synchronized between the applications.