Your cookies store information about websites you have visited. This is not for a malicious purpose, it's so that those websites can remember who you are so you don't have to enter login information every time you visit that page. For instance; when I log in to GMail and I check the "Remember me on this computer" box, the way it remembers me is by storing a cookie on my computer. All a cookie is is a text file with information (they don't store your password so they're not a security risk); they don't slow down your computer or contain viruses or spyware or anything like that. They can be used to trace which websites you've been to to a limited degree, though, if someone hacks or has physical access to your computer. The only consequence of deleting your cookies is that you will have to re-login one time to every website you visit. Your browser cache is an area of your hard drive used to store files associated with webpages you have viewed recently (such as the pages' background images, sounds, and other files you normally have to download when you visit the page) so that you do not have to re-load each individual element of the page when you visit it. It speeds up your web browsing very significantly. It is why it can take minutes to load a given web page but then when you click the back button it is open in a fraction of a second. The only consequence to clearing your cache is that your browsing will seem slower than usual for a while as your browser re-downloads each individual page element of the pages you visit.
In summary, there is NO permanent negative consequence to deleting your cookies or clearing your cache, only a temporary inconvenience.