Every once in awhile, I get someone who refers to an issue with the wrong term, or using a term incorrectly. Worse yet is getting that blank stare when I am talking SEO. So, here are some quick definitions to help clear up any issues for you.
PageRank: It is a measure of “popularity”, “quality” and “connectivity”. Simply, PageRank is calculated by counting up the number of sites that link to the page (not the site) and taking the numerical value of each link received and comparing the total to a numerical range. PageRank is not the main source for the algorithm with Google. PageRank is overrated and if you have “Green Bar Addiction” we are working on a 12-step program for you. 
Sandbox: This is an area that a new site “sits” while it waits to be “trusted” by Google. Many claim that the Sandbox doesn’t exist, including Google. This is real despite what Google says. We will be updating our Getting out the Sandbox article soon. It was interesting that once this term became popular Google called the keyword research tool inside of AdWords the “Sandbox”.
Supplemental Index: This is Google’s “second index” and is a bad place to be. Normally, pages that are here are about to be dropped from the index.
Latent Semantic Indexing (LSI): This is a process where the English language has been converted into numerical data for easy storage and retrieval. Words that are similar have numerical values that are very close. This rewards “theme based” sites. I have seen strong evidence that Google is using LSI in their algorithm. This is how a page can be #1 for a keyword that is not on the page, nor in incoming anchor text.
Hilltop: This is a theory that Google uses this algorithm structure for their results. Hilltop is essentially based on an expert rating system according to Aaron Wall. An example would be if Page A relates to Page B and Page B relates to Page C, then a relationship, or connection between Page A and Page C is assumed. This is strong as this gives topic sensitive sites more of a boost, which would make the SERPs cleaner and more relevant. The areas that are strong for Hilltop are Page Titles, Page Headings, and Bolded Text. Link Popularity is also highly scored. Despite my numerous conversations with Google reps, they will not confirm, nor deny the use of Hilltop in the Google algorithm. It is almost always a “I cannot comment on that.”
Data Center: Refers to the servers where Google stores the data for their index. Google has several data centers, and they constantly test the results and the most consistent is the one that is used. These data centers sit on specific IP addresses rather than domains. When doing testing, Google often serves a different data center to the east coast of the US and another to the west coast. If you are seeing the SERPs “bounce” this means that they are swapping out data centers while they make changes to their algorithm.