Harrison/redo docs (#130)

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

docs/core_concepts.md

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+# Core Concepts
+
+This section goes over the core concepts of LangChain.
+Understanding these will go a long way in helping you understand the codebase and how to construct chains.
+
+## Prompts
+Prompts generically have a `format` method that takes in variables and returns a formatted string.
+The most simple implementation of this is to have a template string with some variables in it, and then format it with the incoming variables.
+More complex iterations dynamically construct the template string from few shot examples, etc.
+
+## LLMs
+Wrappers around Large Language Models (in particular, the `generate` ability of large language models) are some of the core functionality of LangChain.
+These wrappers are classes that are callable: they take in an input string, and return the generated output string.
+
+## Embeddings
+These classes are very similar to the LLM classes in that they are wrappers around models, 
+but rather than return a string they return an embedding (list of floats). This are particularly useful when 
+implementing semantic search functionality. They expose separate methods for embedding queries versus embedding documents.
+
+## Vectorstores
+These are datastores that store documents. They expose a method for passing in a string and finding similar documents.
+
+## Chains
+These are pipelines that combine multiple of the above ideas. 
+They vary greatly in complexity and are combination of generic, highly configurable pipelines and more narrow (but usually more complex) pipelines.