Dedicated IP means your website can run of its own IP, if you dont know what this is or the need for it, you dont really need it.
Cron is a service where you can have scripts set to run at certain times of the day SSI - Server Side Includes (best google it, cant think of the words to explain) SSH - so you can access your space from a SSH Client such as putty, if you dont know what this is or the need for it, it isnt a must have feature anyway.
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SSH alows you to perform commands to a remote machine. Generally, you will not find a shared environment plan that has SSH access included.
I agree with Matt.G the dedicated IP is very important to anyone with a SSL (secure socket layer) which encrypts data transmitted from your website (i.e: credit card information, name, address, etc). In order to use a SSL you need a dedicated IP assigned to your account.
To elaborate on Cron and SSI (as the other two terms were clarified quite nicely), a Cron Tab/Job is basically where you can have a script(s) run at certain times or intervals throughout the day, week, month, year, etc.. all configurable by yourself. Scripts include (but are not limited to) PHP scripts and CGI scripts (if enabled).
SSI is Server Side Includes which is where you can include a bit of code (in a similar format to an HTML comment) where you can have one page include another page. For example, you can use this to include a header/footer file in each of your pages so you only have to worry about updating the content, not worrying about keeping the "design" of your website consistent.
As for SSH, some shared environments will allow this, such as using SSH for creating/coding a website in Ruby on Rails (there are other reasons for which SSH is useful for, as well), but it depends on the individual host and their restrictions/policies. In almost all cases when a host does allow SSH access (either by standard or upon request) it is "jailed shell" so you are limited to some functionality to keep the host's service more secure.