The category of literature searching with which you are probably most familiar is called retrospective searching. Typically a retrospective search begins from a particular article that you have "in-hand", often the result of a search of Chemical Abstracts followed by retrieval of the actual article.
In that article, you see references to other previously published papers. Scientists are positively manic about including in their papers lots of references to other related work. This is done for a number of reasons. Foremost among these is the validation of the present work. It shows that the present work is founded upon the basis of previously peer reviewed publications. This practice also saves time in that one does not have to spend excess ink describing what has already been described elsewhere in the literature.
Some of those references seem relevant to you, so you proceed to retrieve them. Reading through those earlier papers, you encounter still earlier references that you deem useful. So you go to the library to add them to your growing pile of relevant literature articles. Scientists' filing cabinets are stuffed full of such things.
The process of collecting articles in this fashion is termed a retrospective search. The articles are found in a backwards-through-time chronological manner. They have to be because an author can't cite articles that have yet to be published (though you will find an occasional "in press" notation in footnotes).
In contrast to the retrospective search, there is the forward search. A forward search is a search from some specific point in time into the future (future being relative to that point in time). The Science Citation Index (SCI), published by the Institute for Scientific Information (ISI), is perhaps the most authoritative means of performing forward literature searches.
How can you search into the future? It would be nice if the folks at ISI were clairvoyant, but they aren't. The key is the phrase " future being relative to that point in time."
SCI operates from the literature citations in a given published article. Describing how a forward search operates is perhaps best done by contrasting it with a retrospective search.
In a retrospective search you start with an "in-hand" article (presumably an article in which you are interested) as the source and use the references (footnotes) found therein as the object of the search (e.g., go get the reference cited in footnote 5).
In a forward search database, you start with the article "in-hand" as the source, but then ask the database if there are any more recent publications that have cited the "in-hand" paper in their footnotes. If there are, then presumably they are related to the topic of the "in-hand" paper, and therefore might be of interest to you. So you go get them.
In a Retrospective Search, the Citing Article is the source ("in-hand") and the Cited article is the target of the search. In a Forward Search, the Cited Article is the source ("in-hand") and the Citing Article is the target of the search.
Relation to Searching in STN's LCA and CA Files:
It is assumed that you are already familiar with all of the various ways that you can search for information in the CA and LCA files of the STN database, as described in Use of STN and STN Databases. If you do not, this document likely will not provide you with all of the needed information to accomplish a search in file SCIsearch.
In particular, this document assumes that you
1. know how to use STN Express, the program that gives you access to the STN databases.2. are familiar with the commands commonly used in STN databases, e.g. search, expand, and display.
3. know how Boolean expressions (AND, OR, NOT), truncations (e.g., ?), fields (e.g., an, au, py), and formats (e.g., bib, cbib, all) operate.
4. know about transcript files and the various ways that they can be printed.
If you are not comfortable with these concepts, it is STRONGLY SUGGESTED that you go back to Use of STN and STN Databases and learn the basics before proceeding.
Background
Relation to Searching in STN's LCA and CA Files
Sample SCI RecordThe Search Command(S) operatorFields and Formats in SCIsearch
Fields For the Search CommandDefining the Cited Article:(S) proximity operator